Preheat oven to 375°F (190°C). Generously grease a 12-cup muffin tin with butter or non-stick cooking spray.

Heat olive oil in a large skillet over medium heat. Add the chopped onion and cook until softened, about 3-4 minutes. Add the minced garlic and sliced mushrooms and cook until the mushrooms have browned and released their liquid, about 5-7 minutes.

Add the fresh spinach to the skillet and cook, stirring, until it has completely wilted, about 2 minutes. Remove from heat and set aside to cool slightly.

In a large bowl, whisk the eggs until well-beaten. Whisk in the heavy cream, salt, pepper, and nutmeg until fully combined.

Stir the cooled mushroom and spinach mixture into the eggs. Add the shredded Gruyère and grated Parmesan cheese, and mix until everything is evenly distributed.

Divide the mixture evenly among the 12 prepared muffin cups, filling each about three-quarters full.

Bake for 20-25 minutes, or until the muffins are puffed, golden, and a knife inserted into the center comes out clean.

Let the quiche muffins cool in the tin for 5 minutes before running a knife around the edges to loosen and transferring them to a wire rack to cool further. Serve warm or at room temperature.