Preheat the oven to 392°F/200°C. If using a puff pastry sheet: Roll it into a tight log about 0.7 inch (2 cm) thick. Mark 12 equal slices (about 1 inch each).
If using puff pastry dough: Shape into a log or square and cut into 12 equal pieces.
Place each piece in a muffin tin, pressing it to the bottom and sides, halfway up the cups.
Chill the tin with puff pastry in the refrigerator while preparing the mushroom filling.
Clean and slice the mushrooms into smaller pieces and crush the garlic. Dice the cheese into small cubes.
Heat olive oil in a frying pan. Add the mushrooms and a pinch of salt.
Add garlic after 3 minutes, cooking until mushrooms brown and release water.
When done, sprinkle with black pepper.
Transfer to a bowl, add diced cheese and chopped parsley, and mix.
Fill each puff pastry cup with mushroom mixture, being careful not to overfill.
Add a small dollop of sour cream on top of each tartlet.
Bake at 392°F/200°C for 12 minutes, until golden and crispy.
Top with fresh parsley and grated Grana Padano or Parmesan before serving. Serve and enjoy warm.
Bon Appetit!