The active dry yeast produces a hearty dough that holds up well to the savory nature of this dish. The natural sugars in the cooked onions impart a sweet balance to the sherry, thyme, and custard.
Ingredients
Dough
- 1 teaspoon active dry yeast
- Pinch of sugar
- ¼ cup warm water (about 100°F., or just warm to the touch)
- 1⅓ cups all-purpose flour, plus more as needed
- 1 large egg
- ½ teaspoon kosher salt
- 2 tablespoons (¼ stick) unsalted butter, at room temperature, cut into pieces
- Olive oil, for the bowl
Filling
- 2 tablespoons (¼ stick) unsalted butter
- 2 pounds onions, sliced ¼ inch thick (about 8 cups)
- ½ teaspoon sugar
- Kosher salt
- 1 tablespoon dry sherry
- 1 tablespoon chopped fresh thyme
- 2 large eggs
- 1 cup half-and-half
- ½ cup freshly grated Parmigiano-Reggiano
- Cooking spray
How to Make It
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To make the Dough
- In a spouted measuring cup, mix together the yeast and sugar in the water. Let proof until bubbly, about 5 minutes.
- In the bowl of an electric mixer fitted with the paddle attachment, combine the flour, egg, salt, and yeast mixture. Mix on medium speed until combined. Increase the speed to medium high and add the butter, a few pieces at a time, until all is incorporated. Mix another 2 to 3 minutes on medium high to make a smooth but sticky dough. The dough will almost clean the sides of the bowl; add a few more tablespoons of flour, if necessary, to get to this point. Scrape the dough onto a floured work surface, knead once or twice, and transfer to an oiled bowl. Let rise at room temperature until doubled in bulk, about 1 hour. Make the Filling
- While the dough rises, In a large skillet over medium heat, add the butter. When melted, add the onions and sugar and season with salt. Cook, stirring occasionally, until the onions are deep golden brown and very soft, about 45 minutes. Add the sherry and thyme and cook until the sherry is absorbed, 1 to 2 minutes. Remove from the heat and allow the onions to cool to room temperature.
- When you are ready to make the tart, preheat the oven to 350°F. In a large bowl, whisk together the eggs, half-and-half, and cheese and season with salt. Punch down the dough and on a floured work surface roll the dough into a 12-inch circle. Fit the dough into an 8- or 9-inch deep-dish pie plate sprayed with cooking spray and trim the dough so it is flush with the edge of the pie plate. Spread the cooled onions in the bottom and pour the custard over.
- Bake until the tart is just set in the center and the crust is dark golden brown all over, about 40 minutes. Cool on a rack and serve warm or at room temperature.
