Preheat oven to 375°F. Coat a muffin tin with nonstick cooking spray. In a medium bowl (ideally with a spout), beat eggs and salt and pepper to taste (I like 1/2 teaspoon salt and 1/4 teaspoon pepper). Set aside.
In a large skillet over medium-high heat, heat oil until shimmering. Add mushrooms and ¼ teaspoon salt and cook until softened and the mushrooms have released most of their liquid, about 5 minutes.
Using a slotted spoon, remove mushrooms from skillet to a mixing bowl, leaving any remaining oil and liquid in the skillet.
To the skillet with the liquid, add spinach and 1/4 cup of water. Cook spinach until wilted, about 5 minutes.
Add the spinach to the bowl with the mushrooms. Stir in the cheese. Divide the mixture evenly among 12 muffin cups.
Divide the beaten egg mixture among the muffin cups, filling each nearly to the top. Bake until a toothpick inserted comes out clean, about 25 minutes.
Cool slightly before removing from muffin tins (the egg cups should come out easily). Serve hot or at room temperature. Store leftovers in the refrigerator and use within 4 days.
Yield: Makes 12 egg muffins
Storage: Store leftovers covered in the refrigerator for up to 4 days.
Freezer:Cool and transfer to a single layer on a plate. Freeze until solid, at least 30 minutes, then transfer to a freezer-safe bag. Freeze up to 2 months. Microwave them straight from the freezer until heated through, about 1 minute.