Place the flour and salt into the bowl of a food processor. Pulse a couple of times.
Add the butter all at once and pulse about 10 times until the butter is the size of small peas. Add the vinegar and 2 tablespoons of water and pulse a few times. Test the dough by pinching it. If it sticks together well when pinched, you’ve added enough water. If not, add additional water.
Dump the dough mixture, including loose flour, onto a work area. Fraisage, or smear the dough with the palm of one hand. Do this three times. This helps to create the layers of a buttery crust.
Form into a disk, wrap in wax paper or plastic, and chill in the refrigerator for 30 minutes.
Roll out to 1/8″ thickness between two pieces of wax paper. Place in a quiche dish or pie plate. Chill until you’re ready to bake.
See photos and a more detailed explanation in my post on making a fool-proof crust.
Preheat the oven to 425˚F. 'Dock' the shell by pricking the bottom of the shell with a fork numerous times. This helps prevent the shell from billowing upwards off the dish/plate in the oven. Bake the shell for 10 minutes. It will be very lightly browned. If the pastry starts to billow up, just poke it a few more times with the fork, and press it down with a towel. Another option is to 'blind bake' it by filling it with pie weights or dried beans to weight it down.
Preheat the oven to 350˚F.
Melt the butter in a medium saute pan over medium heat. Add the leeks, shallot, and chopped asparagus to the pan and saute until the leeks begin to soften. The asparagus will be only slightly tender at best. Spread on the base of the pre-cooked quiche shell.
Combine the eggs, cheeses, milk, mustard, lemon zest, chives, salt, pepper and thyme in a medium bowl. Whisk until thoroughly mixed. Pour over the sauteed vegetables.
Toss the asparagus spears that have been sliced in half lengthwise in a little olive oil and lemon juice and arrange across the top of the quiche.
Bake for 40 minutes or until set. Remove from the oven and let sit for 15 minutes before serving. This additional sitting time helps the eggs to set up.