Begin by preheating your oven to 350°F (175°C). This ensures that your muffins bake evenly and come out perfectly puffed.
Lightly grease a muffin tin with cooking spray or a small amount of olive oil to prevent sticking. You can also use silicone muffin liners for easy removal.
In a skillet, heat 1 tablespoon of olive oil over medium heat. Add the chopped yellow onion and cook until translucent, about 2-3 minutes. Then, add the diced green bell pepper and chopped brown mushrooms, cooking for another 3-4 minutes until softened. Finally, stir in the chopped spinach and cook until wilted, about 1 minute. Remove from heat and let cool slightly.
In a large mixing bowl, crack 8 eggs and whisk them together until well combined. Season with 1/4 teaspoon sea salt and 1/2 teaspoon ground black pepper, mixing well.
Once the sautéed vegetables have cooled a bit, fold them into the egg mixture. Stir in the 1/2 cup of shredded cheddar cheese until everything is evenly distributed.
Using a ladle or measuring cup, pour the egg and vegetable mixture into each muffin cup, filling them about 3/4 full to allow for rising.
Place the muffin tin in the preheated oven and bake for 20-25 minutes, or until the egg muffins are set in the center and lightly golden on top.
Once baked, remove the muffin tin from the oven and let it cool for a few minutes before gently running a knife around the edges to remove the muffins. Serve warm or allow to cool completely before storing in an airtight container.
