Alsace bakers slow-cook onions for 45 minutes until they vanish into sweetness, transforming humble bulbs into this custard-rich 18th-century tart.
Onion Tart
Indulge in the irresistible flavors of this French onion tart, a delightful dish that's both simple to whip up and utterly satisfying! With caramelized onions, creamy cheese, and a buttery crust, this savory treat is perfect for any occasion. Get ready to impress your guests with a slice of pure deliciousness!
Prep
15
min
Cook
30
min
Serves
4
people
Level
intermediate
The Story
This Alsatian classic owes its soul to Lorraine's quiche-makers and, oddly, to medieval monks who first slathered mustard on pastry to mask inferior fillings—by the 1700s, Alsace-Lorraine border towns were fighting over whose onion tart reigned supreme, and that mustard layer underneath remains the region's stubborn, pungent signature that France's other provinces still haven't properly nicked.
Regional Twist
Over in Alsace's Colmar region, cooks swap plain mustard for Alsatian grain mustard spiked with local Riesling, and toss in a handful of smoked lardons alongside the six onions, giving the tart a boozy, smoky backbone that Lorraine's purists consider borderline heretical.
📝 Ingredients
Serves 4🥬Fresh Produce(2)
- 6 onions, finely sliced
- pepper
🥛Dairy & Eggs(2)
- butter
- 2-3 eggs
🫙Pantry Staples(2)
- 1 tablespoon of flour (15 ml)
- a bit of oil
🧂Spices & Seasonings(1)
- salt
🍯Sauces & Condiments(1)
- mustard
📦Other(1)
- a pie crust
Nutrition (per serving)
Calories
398
Protein
8g
Carbs
39g
Fat
24g
👨🍳 Instructions
- 1
Slice 6 onions finely and sauté them in a pan with a bit of oil and butter, seasoning with salt and pepper, for 20 to 30 minutes until nicely colored.
- 2
Add 1 tablespoon of flour to the onions, mix, and cook for an additional 2-3 minutes. Let it cool off the heat.
- 3
In a bowl, whisk together 2-3 eggs, then mix in the cooled onions.
- 4
Pre-bake a pie crust for 15-20 minutes at 200°C (392°F).
- 5
Spread mustard on the bottom of the crust, pour in the onion mixture, smooth it out, and bake for 30 minutes at 200-220°C (392-428°F).
💡 Pro Tips
- ✓Use a 1:1 oil-to-butter ratio (e.g., 1 tbsp each) for caramelizing onions because oil raises the smoke point to ~350°F while butter adds flavor, preventing burning during the 20-30 minute cook.ingredient1:1 ratio, 350°F smoke point
- ✓Cook onions uncovered on medium heat (not high) for even caramelization; the Maillard reaction requires 300-330°F surface temperature and takes 25-35 minutes for 6 onions to reach deep golden color without scorching.technique300-330°F, medium heat, 25-35 minutes
- ✓Add flour after caramelization is complete and cook for exactly 2-3 minutes to create a roux that binds moisture in the custard; undercooking leaves raw starch taste, overcooking breaks down the thickening starch chains.timing2-3 minutes
- ✓Spread mustard on the warm pre-baked crust before adding filling because its acidity (pH 3.0-3.5) creates a barrier that prevents soggy bottoms by inhibiting water absorption into the crust's starch granules.techniquepH 3.0-3.5
- ✓Bake the filled tart at 210°C (410°F) for 28-32 minutes rather than 200-220°C range; this ensures the custard sets (eggs denature at 158-183°F depending on protein type) while the top browns evenly without crust over-darkening.timing210°C, 28-32 minutes, 158-183°F custard set point
Share this recipe
Prep
15
min
Cook
30
min
Serves
4
people
Level
intermediate
Share this recipe
Alsace bakers slow-cook onions for 45 minutes until they vanish into sweetness, transforming humble bulbs into this custard-rich 18th-century tart.
Onion Tart
Indulge in the irresistible flavors of this French onion tart, a delightful dish that's both simple to whip up and utterly satisfying! With caramelized onions, creamy cheese, and a buttery crust, this savory treat is perfect for any occasion. Get ready to impress your guests with a slice of pure deliciousness!
The Story
This Alsatian classic owes its soul to Lorraine's quiche-makers and, oddly, to medieval monks who first slathered mustard on pastry to mask inferior fillings—by the 1700s, Alsace-Lorraine border towns were fighting over whose onion tart reigned supreme, and that mustard layer underneath remains the region's stubborn, pungent signature that France's other provinces still haven't properly nicked.
Regional Twist
Over in Alsace's Colmar region, cooks swap plain mustard for Alsatian grain mustard spiked with local Riesling, and toss in a handful of smoked lardons alongside the six onions, giving the tart a boozy, smoky backbone that Lorraine's purists consider borderline heretical.
📝 Ingredients
Serves 4🥬Fresh Produce(2)
- 6 onions, finely sliced
- pepper
🥛Dairy & Eggs(2)
- butter
- 2-3 eggs
🫙Pantry Staples(2)
- 1 tablespoon of flour (15 ml)
- a bit of oil
🧂Spices & Seasonings(1)
- salt
🍯Sauces & Condiments(1)
- mustard
📦Other(1)
- a pie crust
Nutrition (per serving)
Calories
398
Protein
8g
Carbs
39g
Fat
24g
👨🍳 Instructions
- 1
Slice 6 onions finely and sauté them in a pan with a bit of oil and butter, seasoning with salt and pepper, for 20 to 30 minutes until nicely colored.
- 2
Add 1 tablespoon of flour to the onions, mix, and cook for an additional 2-3 minutes. Let it cool off the heat.
- 3
In a bowl, whisk together 2-3 eggs, then mix in the cooled onions.
- 4
Pre-bake a pie crust for 15-20 minutes at 200°C (392°F).
- 5
Spread mustard on the bottom of the crust, pour in the onion mixture, smooth it out, and bake for 30 minutes at 200-220°C (392-428°F).
💡 Pro Tips
- ✓Use a 1:1 oil-to-butter ratio (e.g., 1 tbsp each) for caramelizing onions because oil raises the smoke point to ~350°F while butter adds flavor, preventing burning during the 20-30 minute cook.ingredient1:1 ratio, 350°F smoke point
- ✓Cook onions uncovered on medium heat (not high) for even caramelization; the Maillard reaction requires 300-330°F surface temperature and takes 25-35 minutes for 6 onions to reach deep golden color without scorching.technique300-330°F, medium heat, 25-35 minutes
- ✓Add flour after caramelization is complete and cook for exactly 2-3 minutes to create a roux that binds moisture in the custard; undercooking leaves raw starch taste, overcooking breaks down the thickening starch chains.timing2-3 minutes
- ✓Spread mustard on the warm pre-baked crust before adding filling because its acidity (pH 3.0-3.5) creates a barrier that prevents soggy bottoms by inhibiting water absorption into the crust's starch granules.techniquepH 3.0-3.5
- ✓Bake the filled tart at 210°C (410°F) for 28-32 minutes rather than 200-220°C range; this ensures the custard sets (eggs denature at 158-183°F depending on protein type) while the top browns evenly without crust over-darkening.timing210°C, 28-32 minutes, 158-183°F custard set point