01 - Preheat your oven to 375°F (190°C). Lightly grease a 12-cup muffin tin or line with silicone liners for easier removal.
02 - Heat olive oil in a skillet over medium heat. Add the red onion, bell pepper, and zucchini. Sauté for 3-4 minutes until they begin to soften. Stir in the baby spinach and cook just until wilted. Remove from heat and allow to cool slightly.
03 - In a spacious mixing bowl, thoroughly whisk together the eggs, milk, salt, black pepper, and garlic powder until the mixture is uniform. If using cheese, fold it into the egg mixture.
04 - Distribute the sautéed vegetable mixture evenly among the prepared muffin cups. Carefully pour the egg mixture over the vegetables, filling each cup approximately three-quarters full.
05 - Place the muffin tin in the preheated oven and bake for 18-20 minutes, or until the muffins are set and display a light golden hue on their surface.
06 - Remove the muffins from the oven. Let them cool in the tin for 5 minutes before gently running a knife around the edges of each muffin to release them. Serve warm or at room temperature.