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

In a large skillet, heat olive oil over medium heat. Add the shallot and cook until softened. Add mushrooms and cook until browned. Stir in the garlic until fragrant. Add the spinach and cook until just wilted. Remove from heat and squeeze out all excess liquid.

In a large bowl, whisk the eggs, heavy cream, milk, salt, pepper, and nutmeg until smooth. Stir in the shredded Gruyere cheese and the cooled mushroom and spinach mixture.

Pour the egg mixture evenly into the prepared muffin cups, filling each about 3/4 full.

Bake for 20-25 minutes, or until the muffins are set, puffed, and lightly golden. A toothpick inserted into the center should come out clean.

Let the quiche muffins cool in the tin for 5-10 minutes before running a knife around the edges and removing them. Serve warm or at room temperature.