Preheat oven to 400 degrees.
Line a cookie sheet with parchment paper or use a Silpat mat.
You can do this a couple of different ways – either use a whole sheet of puff pastry or cut a piece in half lengthwise to make two smaller tarts. Dust your counter top with flour and roll out (to about 10x12 inches if using an entire sheet, or about 7x10 if using two halves).
Place puff pastry in center of the sheet. Using a paring knife, make a shallow cut around the entire sheet of pastry about ½ inch from the edge, then fold over to create a crust/border. Using a fork, prick holes all over the entire pastry, including the border.
In a medium sized bowl, combine the ricotta, mascarpone, and gruyere cheese. Stir in herbs and lemon zest. Taste for seasoning and add 1/2 -1 tsp. salt. If using half sheets, spread about ½ cup of cheese mixture evenly over top of pasty, leaving about ½ inch border all around. If using an entire sheet, use about 1 cup of cheese mixture.
For the Asparagus Tart: Arrange asparagus over top of cheese mixture in layout of your choice.
For the Mushroom Tart: Sprinkle sautéed mushrooms over top of cheese mixture.
Bake tarts for about 25 minutes, or until tart shell has darkened, cheese is bubbling, and asparagus is crisp-tender.
Remove from oven and allow to cool for about 15 minutes before serving.
COOKS NOTES
Amazingly enough, this tart is as good at room temperature as it is hot out of the oven.
If you don’t eat a whole tart in one sitting (as if), you can re-crisp it by putting in a 350-degree oven for a few minutes until the bottom crust crisps up again.