Alsatian Caramelized Onion Tart

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A rustic specialty from the Alsace region in northeastern France, this savory tart features a buttery shortcrust shell filled with slowly caramelized yellow onions bound by a light egg-and-cream custard. It is the soul of Alsatian bistro cooking: simple, deeply golden, and unapologetically rich. Serve warm with a crisp green salad and a glass of dry Riesling.

Nutrition Facts

Per serving (estimated)

• 470 kcalCalories
• 32 gFat
• 17 gSaturated Fat
• 36 gCarbs
• 3 gFiber
• 9 gSugar
• 10 gProtein
• 480 mgSodium
• 280 mgPotassium
• 95 mgCalcium
• 2 mgIron
• 8 mgVitamin C
• 115 mcgVitamin A

Ingredients

For the shortcrust pastry

• 1 1/4 cups (160 g) all-purpose flour
• 1/2 teaspoon fine sea salt
• 1/2 cup (115 g) cold unsalted butter, cubed
• 1 large egg yolk
• 2 to 3 tablespoons ice water

For the caramelized onion filling

• 2 lbs (900 g) yellow onions, halved and thinly sliced
• 3 tablespoons unsalted butter
• 1 tablespoon olive oil
• 1 teaspoon granulated sugar
• 1/2 teaspoon salt
• 4 oz (115 g) smoked lardons, optional

For the egg-cream custard

• 3 large eggs
• 3/4 cup (180 ml) heavy cream
• 1/4 cup (60 ml) whole milk
• 1/2 teaspoon fine sea salt
• 1/4 teaspoon freshly ground white pepper
• Pinch of freshly grated nutmeg
• 1/3 cup (35 g) grated Gruyère, optional

Directions

1. Make the pastry: pulse the flour, salt, and butter in a food processor until pebbly. Add the egg yolk and 2 tablespoons of ice water and pulse just until the dough comes together; add the remaining water if needed. Form into a disc, wrap, and chill for 30 minutes.
2. Roll the dough on a floured surface into a 12-inch round and press into a 10-inch tart pan with a removable bottom. Trim the edges, prick the base all over with a fork, and chill for 15 minutes. Blind bake at 375°F (190°C) for 12 minutes, then remove the parchment and weights and bake 5 minutes more until just set.
3. Meanwhile, melt the butter with the olive oil in a large skillet over medium-low heat. Add the sliced onions, sugar, and salt, and cook gently, stirring often, for 30 to 40 minutes until the onions are deep amber and very soft. If using lardons, cook them separately in a dry pan until crisp, drain, and stir into the onions.
4. Spread the caramelized onions evenly in the pre-baked tart shell and sprinkle the lardons over the top if using.
5. In a bowl, whisk the eggs, cream, milk, salt, white pepper, and nutmeg until smooth. Pour the custard slowly over the onions, allowing it to seep through.
6. Scatter the grated Gruyère over the surface if using, which adds a nutty Alsatian note.
7. Bake at 375°F (190°C) for 30 to 35 minutes, until the custard is just set in the center and the top is golden brown. A knife inserted near the center should come out clean.
8. Let the tart rest at room temperature for 10 minutes to finish setting, then unmold and serve warm or at room temperature with a simple green salad dressed in vinaigrette.

Cook’s Notes

• Patience is the secret: cook the onions over medium-low heat so they caramelize rather than scorch, building deep sweetness without burning.
• For an authentic touch, add a pinch of caraway seeds to the custard, a classic Alsatian seasoning.
• The tart is best served the day it is baked but keeps refrigerated for 2 days; rewarm gently in a 300°F oven.
• Substitute thick crème fraîche for the heavy cream for a tangier, more traditional Alsatian flavor.
• Blind baking the crust is essential to keep the bottom crisp under the wet custard filling.

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