Preheat your oven to 350°F (175°C). Lightly spray a 12-cup muffin tin with non-stick cooking spray or line with paper liners for easy removal.

Finely chop the fresh spinach and red bell pepper (if using). If you're using frozen spinach, ensure it's completely thawed and squeeze out all excess water thoroughly to prevent watery muffins.

In a large mixing bowl, whisk the eggs vigorously until the yolks and whites are fully combined and slightly frothy. This helps create a lighter texture.

Add the unsweetened almond milk (or milk of choice), finely chopped spinach, diced red bell pepper (if using), shredded cheese (if using), salt, black pepper, garlic powder, and onion powder to the whisked eggs. Stir gently but thoroughly until all ingredients are well combined and evenly distributed.

Evenly divide the egg mixture among the 12 prepared muffin cups, filling each approximately 3/4 full. Avoid overfilling to prevent spillage during baking.

Place the muffin tin in the preheated oven and bake for 18-22 minutes, or until the egg muffins are fully set, puffed, and lightly golden brown around the edges. You can check for doneness by inserting a toothpick into the center; it should come out clean.

Remove the muffin tin from the oven and let the muffins cool in the tin for 5 minutes. This allows them to firm up slightly. Then, carefully transfer them to a wire rack to cool completely before serving or storing.