Preheat the oven to 350°F (175°C).

Lightly coat a 12-cup standard muffin tin with non-stick cooking spray to ensure easy removal.

Distribute the diced red bell pepper, chopped baby spinach, and red onion evenly across the bottom of the 12 muffin cups.

In a medium mixing bowl, whisk together the liquid egg whites, sea salt, ground black pepper, and garlic powder until the seasoning is evenly distributed.

Carefully pour the egg white mixture into each muffin cup, filling each cup until approximately 3/4 full to allow for a slight expansion during baking.

Place the muffin tin on the center rack of the oven and bake for 22 to 25 minutes, or until the egg whites are firm to the touch and the centers are set.

Remove the tin from the oven and allow the muffins to cool for 5 minutes.

Run a thin silicone spatula or knife around the edges of each muffin to release them from the tin before serving.