Step 1: Preheat your oven to 350°F (175°C) to ensure even baking.
Step 2: In a skillet, heat olive oil over medium heat. Add the chopped onion and sauté until translucent, about 3-4 minutes. Add green bell pepper and brown mushrooms, cooking for another 5 minutes. Finally, add spinach and cook until wilted. Remove from heat and cool slightly.
Step 3: In a large mixing bowl, crack the eggs and whisk until fully combined. Season with sea salt and black pepper.
Step 4: Gently fold the sautéed vegetables into the beaten eggs, then stir in cheddar cheese until evenly distributed.
Step 5: Grease your muffin tin and pour the egg mixture into each cup, filling them about 3/4 full.
Step 6: Bake for 18-20 minutes, or until set and lightly golden on top.
Step 7: Cool in the tin for 5 minutes before transferring to a wire rack. Enjoy warm or store for later!