Leek, 1 small, white part only, halved, rinsed, and thinly sliced
Milk, ¼cup (2fl oz/60ml)
Salt and freshly ground pepper
Prepare the pastry
Preheat the oven to 400°F(200°C). Line a rimmed baking sheet with parchment (baking) paper. Lay the puff pastry on the baking sheet. Fold over the edges to make a 1-inch (2.5-cm) border, overlapping the pastry at the corners and pressing it lightly. Inside the border, prick the pastry all over with a fork.
Fill and bake the tart
Sprinkle half of the cheese evenly over the pastry inside the border. Lay the asparagus in a row from one side of the pastry to the other. Sprinkle with the leeks. Bake for 15 minutes. Meanwhile, in a bowl, beat the eggs, milk, ½teaspoon salt, and pepper to taste until well combined. Pour the egg mixture over the asparagus and leeks and sprinkle with the remaining cheese. Bake until the pastry is puffed and golden, about 10 minutes. Let stand for 10 minutes. Cut into pieces and serve.