Preheat your oven to 350 degrees F and lightly coat a 6-cup muffin tin with nonstick spray. Divide half of the green asparagus, bell pepper, and tomatoes among the cups in the muffin tin.
In a large bowl add the eggs, chopped parsley, and season with a pinch of salt, pepper, and cayenne pepper. Whisk gently until combined.
Fill each muffin cup with the egg mixture, top with the other half of the vegetables, and sprinkle the feta evenly over the top.
Bake the muffins for 20- 25 minutes in the preheated oven until the eggs are set. Remove the baked egg muffins from the oven and let cool for a few minutes.