Upside Down Mushroom Cheese Tartlet

Martha Stewart had these on the Today Show. An easy way to use MUFFIN PANS. I could think of other things  to add to this recipe as well, like bacon, onion, peppers? Anyway it's easy and really good.

• 1 sheet frozen puff pastry, thawed and cut into six 3-inch rounds
• 1 tablespoon extra-virgin olive oil, plus more for muffin pan
• 1 large shallot, diced small
• 1 pound cremini mushrooms, sliced
• Coarse salt and pepper
• 2 teaspoons fresh thyme leaves, plus more for serving
• 3/4 cup grated Gruyere cheese

Preheat oven to 375 degrees. Place puff pastry rounds in a single layer on a parchment-lined baking sheet and refrigerate while mushrooms cook.
Filling 
In a large skillet, heat over medium high: 1 tablespoon extra-virgin olive oil. Then add 1 large shallot, diced small, cook, stirring, until soft for 3 minutes. Then add 1 pound cremini mushrooms, sliced, cook, stirring, until soft and browned, 10 minutes. Season with salt and pepper. Stir in 2 teaspoons fresh thyme leaves. Remove skillet from heat. Muffin tins Lightly oil 6 jumbo muffin cups with olive oil. Divide among cups the mushroom mixture and 3/4 cup grated Gruyere cheese (2 ounces). Top each mixture with chilled round pastry. Bake until pastry is golden brown and puffed, about 25 minutes, rotating pan halfway through. Run a small knife around cups to loosen tartlets. Place a rimmed baking sheet or large plate over pan and invert to release tartlets. Sprinkle with more thyme. Serve warm. Serves 6.