Preheat the oven to 350 degrees F. Generously grease a muffin tin with oil or cooking spray, or line the tin with silicone muffin liners.
In a skillet over medium high heat, heat the olive oil and add the diced scallions and mushrooms. Saute for 5 to 7 minutes or until the veggies are tender. If there is any excess water from the mushrooms, you can drain it off.
While the veggies are cooking, whisk the eggs in a large mixing bowl. Add the feta, pepper, and salt, and stir until combined.
Prepare the muffins by adding a spoonful of veggies to the bottom of each muffin cup then filling the rest of the space with the egg mixture.
Bake for 20 to 25 minutes until the eggs are set and a toothpick inserted in the center comes out clean. Enjoy!
Storage: Egg muffins can last for up to five days in an airtight container in the fridge. To freeze them, let the muffins cool completely. Wrap each one in plastic wrap, then place the wrapped muffins in a large freezer-friendly bag. Store in the freezer for up to 3 months.
Reheating (from the fridge or freezer): Unwrap egg muffins and place on a microwave-safe plate. Reheat for 30 to 60 seconds on high or until they have reached your desired temperature.
Variations: Feel free to sub other types of veggies for the mushrooms and scallions (I like bell peppers and leafy greens). You can also use different types of cheese.