Prepare the vegetable mix: Wash and finely chop the bell peppers, onion, and spinach. If using other vegetables, make sure they are cut small so they cook evenly in the muffins. Set aside in a bowl.

Whisk the eggs: In a medium-sized mixing bowl, crack all six eggs. Add the milk, salt, black pepper, and garlic powder if using. Whisk together until fully combined and slightly frothy. This helps make the egg muffins light and fluffy.

Combine eggs and vegetables: Add the chopped vegetables and shredded cheese (if using) to the whisked eggs. Stir gently to ensure even distribution of vegetables in every bite.

Prepare the muffin cups: Lightly grease silicone muffin cups or a metal muffin pan with cooking spray or a small amount of olive oil. Pour the egg mixture evenly into six cups, filling each about 3/4 full to allow room for rising.

Preheat the air fryer: Preheat your air fryer to 320°F (160°C) for 3–5 minutes. Preheating ensures even cooking and prevents sticking.

Cook the egg muffins: Carefully place the filled muffin cups into the air fryer basket. Cook at 320°F (160°C) for 12–14 minutes. Check for doneness by gently inserting a toothpick in the center of a muffin—if it comes out clean, they are ready. The tops should be lightly golden and fully set.

Cool and serve: Allow the muffins to cool for 2–3 minutes before removing them from the cups. This helps them hold their shape. Serve warm as a healthy breakfast, snack, or protein-packed addition to lunchboxes.

Serving suggestions: You can enjoy them plain, or serve with avocado slices, a side of fresh fruit, or a drizzle of hot sauce for extra flavor. They can also be stored in an airtight container in the fridge for up to 4 days and reheated in the air fryer or microwave.