3/4 lb. bacon, cut into medium diceIn a medium skillet over medium-high heat, cook the bacon until browned and crispy, 6 to 8 minutes. Using a slotted spoon, transfer the bacon to a plate lined with a paper towel. Pour off all but 1 to 2 Tbs. of fat in the skillet. Set the skillet over medium heat and cook the leeks, stirring occasionally, until softened, about 5 minutes. Let cool slightly.
3 cups medium-diced leeks, washed and drained (about 3 leeks, white and light green parts only)
1-1/4 cups half-and-half
1 cup grated extra-sharp Cheddar (4 oz.)
2 large eggs
2 large egg yolks
2 Tbs. chopped fresh thyme leaves
1 tsp. kosher salt
1/2 tsp. freshly ground black pepper
1/4 tsp. ground nutmeg
2 1.1-lb. packages frozen puff pastry sheets, thawed according to package directions (preferably Pepperidge Farm brand)
Flour as needed for rolling out the dough
While the leeks cool, combine the half-and-half, Cheddar, eggs, egg yolks, thyme, salt, pepper, and nutmeg in a medium bowl. Add the cooled leeks and the bacon and stir to combine.
Lightly spray four mini muffin tins (or two if your tins have two dozen cups) with cooking spray.
Working with one sheet of puff pastry dough at a time, use a floured rolling pin to roll the dough on a lightly floured work surface into a 10- by 18-inch rectangle. Stamp out 3-inch circles of dough with a cookie cutter and gently press the rounds into the mini muffin tins, making sure that each round is centered and that the dough extends up to the top of the tin. Fill each with about 1 Tbs. of filling—you can fill right to the top of the dough. Repeat with the remaining dough and filling until all the filling is used. You may not need all the dough; save leftovers for another use. You can bake the quiches immediately or freeze and bake them later (see Make-Ahead Tip, below).
Heat the oven to 400°F and position racks in the top and bottom thirds of the oven. Bake the quiches, switching the position of the tins halfway through baking, until the filling is puffed and the crust is golden brown, about 20 minutes.
To Make Ahead
Unbaked quiches can be frozen for up to a month. Freeze the quiches in the muffin tins for about 2 hours, or until set. Remove them from the tins and transfer them to an airtight container, setting parchment or plastic wrap between layers, or seal them in a plastic bag. To bake, transfer the quiches back to the tins and bake immediately, without thawing. Baking time for frozen quiches will be about 30 to 35 minutes.