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

Heat olive oil in a large skillet over medium heat. Add the chopped mushrooms and cook for 5-7 minutes until they release their moisture and begin to brown. Add the fresh spinach and cook until fully wilted and all excess water has evaporated. Stir in the minced garlic, onion powder, salt, and pepper, cooking for one more minute until fragrant. Remove from heat.

In a large mixing bowl, whisk the eggs thoroughly. Whisk in the milk or heavy cream until smooth. Stir in 3/4 cup of the shredded cheddar cheese.

Distribute the cooked spinach and mushroom mixture evenly among the 12 muffin cups. Pour the egg mixture over the vegetables, filling each cup about 3/4 full.

Sprinkle the remaining 1/4 cup of cheese over the top of the muffins. Bake for 20-25 minutes, or until the centers are set (a knife inserted comes out clean) and the tops are lightly golden.

Let the quiche muffins cool in the tin for 5-10 minutes before carefully removing them. Serve warm or allow to cool completely for storage.