This tart, which comes from baker Nick Malgieri, uses a salty, smoky ham known as speck, a traditional ingredient in this tart; bacon is a good substitute.
FOR THE CRUST:
1½ cups flour
½ tsp. baking powder
¼ tsp. kosher salt
8 tbsp. unsalted butter, chilled and cubed
FOR THE FILLING:
3 oz. speck or bacon, finely chopped
3 tbsp. unsalted butter
1½ lbs. white onions (about 3), finely chopped
Kosher salt to taste
½ cup milk
½ cup heavy cream
3 oz. grated Gruyère or Swiss cheese
Freshly ground black pepper and freshly grated nutmeg
1. Make the crust: Pulse flour, baking powder, and salt in a food processor to combine. Add butter and pulse until pea-size pieces form. Add ¼ cup ice-cold water; pulse briefly. Transfer dough to a floured work surface; form into a disk. Wrap in plastic wrap; chill for 1 hour.
2. Heat oven to 425°. Transfer dough to a floured work surface. Roll dough into a 13" circle; transfer to an 11" fluted tart pan with a removable bottom, pressing into bottom and sides. Trim excess dough; chill for 30 minutes. Prick bottom of dough with a fork; cover bottom with a sheet of parchment paper; fill with dried beans. Bake until crust is set, about 20 minutes. Remove paper and beans; bake until crust is light brown, about 15 minutes. Let cool.
3. Meanwhile, make filling: Cook bacon in a 4-qt. saucepan over medium heat until crisp, 7–8 minutes. Using a slotted spoon, transfer bacon to a plate. Discard bacon fat; add butter. Add onions and season with salt; reduce heat to low, cover, and cook, stirring occasionally, until soft, about 10 minutes. Uncover; cook until lightly browned, 10 minutes. Let cool.
4. Whisk milk, cream, and eggs in a bowl; add reserved bacon and onions along with cheese. Season with salt, pepper, and nutmeg. Pour filling into tart; place on a baking sheet. Place tart in oven and reduce heat to 400°. Bake until filling is golden brown and set, 45–50 minutes. Transfer tart to a wire rack to let cool completely before slicing and serving.