Thaw puff pastry in the refrigerator overnight.

Preheat oven to 400F/205C. Roll out pastry on a lightly floured surface to about 1/8” thick (about 12” X 16”). Using a round cookie cutter, cut 20 rounds from the pastry. Place these on a baking sheet covered with parchment. Using a fork, prick pastry to limit its rising. Bake for about 10 minutes, then remove from the oven and carefully punch pastry down using the fork again. Return to the oven and bake until browned, about 10 more minutes (keep an eye on them; you want them cooked but not too brown). These can be made up to two days ahead of time.

To prepare parsnips
Place peeled, chopped parsnips in a pot and add milk. Add water to cover the parsnips if needed. Cook until parsnips are soft, about 20 minutes. Strain off parsnips, retaining the milk. Place parsnip in a blender and add enough milk to purée. Add butter and parmesan, and then season to taste with salt and pepper. The purée can be cooled and refrigerated the day before you use it.

Mushrooms and assembly
Preheat the oven to 350F/177C. Slice mushrooms about 1/8” thick and sauté in olive oil until just browned, about 5 minutes. Remove and place on a paper towel. Meanwhile, reheat purée gently until warm (you can use the microwave here). Transfer purée to a piping bag. Place the puff pastry rounds on a parchment-lined baking sheet and pipe about 1 tbsp (15 mL) of purée into each. Top purée with sautéed mushrooms and bake until heated through, about 10 minutes. Carefully remove from the baking sheet onto a platter and top with pea shoots or microgreens. Serve warm.