Preheat the oven to 375°F (190°C). Grease a 12-cup muffin pan with olive oil.
In a skillet, heat olive oil over medium heat. Add mushrooms and sauté until they release their moisture and become browned, about 5 minutes.
In a large bowl, whisk together eggs, salt, and pepper. Stir in chopped spinach, sautéed mushrooms, and shredded cheese until well combined.
Divide the egg mixture evenly among the prepared muffin cups, filling each about two-thirds full.
Bake in the preheated oven for 18-20 minutes, or until the egg muffins are set and lightly golden.
Remove from the oven and let cool in the pan for a few minutes. Use a knife to gently release the muffins and serve warm.
Notes
These egg muffins can be stored in the refrigerator for up to 4 days or frozen for up to 1 month. Reheat in the microwave for a quick breakfast option.