Preheat your oven to 350°F (175°C).
In a skillet, heat the olive oil over medium heat. Add the chopped yellow onion and diced green bell pepper. Sauté for about 3-4 minutes until they begin to soften. Then, add the chopped brown mushrooms and continue to cook for another 2-3 minutes. Finally, stir in the chopped spinach and cook until wilted, about 1-2 minutes. Remove from heat and let cool slightly.
In a large mixing bowl, crack the 8 eggs. Add the sea salt and ground black pepper. Use a whisk or fork to beat the eggs until they are well combined and slightly frothy.
Once the sautéed vegetables have cooled a bit, add them to the bowl with the beaten eggs. Add in the shredded cheddar cheese and fold gently until everything is well combined.
Grease the muffin tin with a bit of olive oil or cooking spray to prevent sticking. Pour the egg mixture evenly into each muffin cup, filling them about 3/4 full.
Place the muffin tin in the preheated oven and bake for 18-20 minutes, or until the egg muffins are set and lightly golden on top. A toothpick inserted into the center should come out clean.
Once baked, remove the muffins from the oven and allow them to cool in the tin for about 5 minutes. Then, carefully remove them from the tin and let them cool further on a wire rack.
