Preheat and prep: Preheat your oven to 350°F (175°C) and generously spray a 12-cup muffin tin with cooking spray or lightly brush with olive oil.
Prepare the vegetables: Roughly chop the spinach into bite-sized pieces, dice the red bell pepper into small pieces, and thinly slice the green onions.
Whisk the egg mixture: Crack the eggs into a large mixing bowl and whisk until well combined and slightly frothy, then add the almond milk, garlic powder, black pepper, and salt and whisk again.
Add the vegetables and cheese: Fold in the chopped spinach, diced bell pepper, sliced green onions, and crumbled feta cheese, stirring gently to distribute evenly.
Fill the muffin cups: Using a 1/4 cup measuring cup or ladle, pour the egg mixture into each muffin cup, filling them about 3/4 full.
Distribute ingredients evenly: Use a fork to gently push vegetables down into each muffin cup if they've floated to the top.
Bake: Place the muffin tin in the preheated oven and bake for 18-22 minutes, until the eggs are set in the center and the tops are lightly golden.
Cool and remove: Let the muffins cool in the pan for 5 minutes, then run a butter knife around the edges if needed and gently lift each muffin out.
Serve or store: Enjoy immediately while warm, or let cool completely before storing for meal prep.