Step 1

Heat a large skillet over medium heat and add the butter. Add the leeks, season with salt and pepper and cook, stirring occasionally, until soft and golden brown around the edges, 8 to 10 minutes. Set aside.

Step 2

Preheat the oven to 450˚. Unroll both pie crusts onto a sheet pan. Cut the dough where it overlaps and press the pieces together to make a 10-by-16-inch rectangle, using the scraps you’ve cut to fill in any spaces. Keep the dough as thin as possible. Sprinkle on the fontina in a single layer, leaving a 1-inch border, then top with the leeks. Dollop the ricotta all over the tart and then smooth it out as best as you can with a knife or an offset spatula. Scatter the thyme and the asparagus pieces over the ricotta layer. Lastly, fold in the edges of the dough to make a ½ -inch border all the way around the tart.

Step 3

Whisk the egg and 1 tablespoon water in a small bowl and brush it all around the crust edge. Bake the tart, watching carefully to make sure the crust doesn’t burn, until it is a deep golden brown, 15 to 18 minutes. If the crust is browning too fast, reduce the oven temperature to 425˚.

Step 4

Remove the tart from the oven; allow it to sit for 5 minutes. Cut into pieces and serve.