Preheat the oven to 200 C / 400 F & Prepare a baking tray by lining with parchment paper and greasing with butter or olive oil.
Place the puff pastry flat on the tray. Prick it with a fork all over and blind bake it for 10-12 minutes until it is lightly golden brown in color. Remove it from the oven, and turn the temperature down to 150 C / 300 F
Blanch the asparagus in a saucepan of salted boiling water for 2 minutes, then transfer it to a bowl of cold water to stop them from cooking.
Make the cheese filling by mixing grated cheese, mustard, yogurt, milk, and egg yolks together in a bowl. Season with some salt and pepper.
Spread the mixture evenly over the puff pastry, leaving a margin at the edges. Lay the asparagus spears in an even line on top.
Bake the asparagus tart in the (now cooler) oven for a further 15-20 minutes until set. You'll know it's ready because the cheese filling will have enclosed around the asparagus slightly, and if you jiggle the tray, it will be firm and not still runny in the middle.