YIELD: 8-10 servings
1 (14 to 17 ounce) package frozen puff pastry (preferably all-butter), thawed in the refrigerator
1 pound asparagus, preferably thin
2 large eggs
1/3 cup whole or 2% milk
2 tsp. Dijon mustard
1/4 tsp. kosher salt
1/8 tsp. freshly ground black pepper
1 cup small-dice cooked ham
1 cup grated Gruyère cheese, divided
Arrange a rack in the middle of the oven and heat to 425°F.
Meanwhile, trim the woody stems from the asparagus. Pat the asparagus dry with paper towels. Halve lengthwise if the asparagus is thick; set aside.
If your puff pastry is folded, let sit out at room temperature for about 15 minutes to soften slightly.
Line a large, rimmed baking sheet with parchment paper. Move the parchment to a work surface. Unfold the puff onto the parchment (if your package has 2 sheets, place them side by side, slightly overlapping, so that they fit the baking sheet, and press together). Using a rolling pin, roll it into a rectangle about the size of the baking sheet. Transfer the puff, still on the parchment, to the baking sheet.
Carefully roll about 1/2 inch of each side in on itself toward the center to form the outer rim of the tart. Poke the bottom of the crust with a fork so that the pastry doesn’t puff up. Bake until the edges puff and the top is a pale golden-brown, about 15 minutes.
Meanwhile, place the eggs, milk, Dijon, salt, and pepper in a medium bowl and whisk to combine; set aside.
Place the baking sheet on a wire rack. Sprinkle the crust evenly with the ham and 1/2 of the cheese.
Slowly pour the egg mixture evenly over the crust. Lay the asparagus spears over the egg mixture in a single layer. Sprinkle with the remaining cheese.
Bake until the edges are puffed and the bottom is crisp and golden-brown, 35-40 minutes. If there is moisture on the top of the tart, blot it gently away with paper towels. Cut into pieces and serve warm or at room temperature.