Preheat oven to 350°F (175°C) and generously grease a 12-cup muffin tin or use silicone liners.
In a bowl, whisk eggs until smooth and slightly frothy.
Add milk, salt, pepper, garlic powder, and paprika. Whisk to combine.
Stir in cooled quinoa, cheese, bell peppers, spinach, green onions, and any optional add-ins.
Pour mixture into muffin cups, filling each about ¾ full.
Bake for 20–22 minutes until centers are set and tops are lightly golden.
Cool in the pan for 5 minutes, then loosen with a knife and remove.