Follow these simple steps to bake a batch of perfect Egg White Veggie Muffins:

1. Preheat and Prepare: Preheat your oven to 375°F (190°C). Thoroughly grease a standard 12-cup muffin tin with non-stick cooking spray. Alternatively, line the cups with silicone muffin liners (paper liners are not recommended as egg whites tend to stick firmly to them). This step is critical – don’t skip it or skimp on the grease!
2. Sauté the Vegetables: Heat the olive oil in a medium skillet over medium heat. Add the diced red bell pepper and chopped mushrooms (if using). Sauté for 3-4 minutes until they begin to soften. Add the chopped spinach and cook for another 1-2 minutes, just until the spinach wilts down. Cooking the vegetables first serves two purposes: it enhances their flavour and, more importantly, releases excess moisture, preventing soggy muffins. Remove from heat and let the mixture cool slightly.
3. Combine Wet Ingredients: In a large bowl or a large measuring cup with a spout (for easy pouring), whisk together the liquid egg whites, garlic powder, onion powder, salt, and black pepper. Whisk until well combined, but avoid over-whisking, which can incorporate too much air and make the muffins overly puffy and then collapse.
4. Add Vegetables and Cheese: Stir the slightly cooled sautéed vegetable mixture (peppers, mushrooms, spinach) into the egg white mixture. Add the sliced green onions, diced cherry tomatoes, and grated Parmesan cheese (if using). Stir gently until everything is evenly distributed.
5. Fill Muffin Cups: Carefully pour or ladle the egg white and vegetable mixture into the prepared muffin cups. Fill each cup about three-quarters full. Avoid overfilling, as the muffins will puff up slightly during baking. Ensure the vegetables are distributed evenly among the cups.
6. Bake: Place the muffin tin in the preheated oven. Bake for 18-22 minutes, or until the muffins are set, puffed up, and lightly golden around the edges. The exact time will depend on your oven and the size of your muffin cups. You can check for doneness by inserting a toothpick into the center of a muffin; it should come out clean. Be careful not to overbake, as this can result in rubbery muffins.
7. Cool: Once baked, carefully remove the muffin tin from the oven. Let the muffins cool in the tin for about 5-10 minutes. This allows them to set further and makes them easier to remove.
8. Remove and Serve/Store: Gently run a thin knife or spatula around the edge of each muffin to loosen it, then lift them out. If using silicone liners, they should pop out easily once slightly cooled. Serve warm, or allow them to cool completely before storing.