01 - Preheat oven to 350°F. Lightly grease a 12-cup muffin tin using olive oil.
02 - In a large mixing bowl, whisk eggs, milk, salt, black pepper, and smoked paprika until fully combined.
03 - Fold in chopped spinach, diced bell pepper, halved cherry tomatoes, finely chopped red onion, shredded cheddar, and crumbled feta cheese if desired. Stir gently until evenly distributed.
04 - Evenly divide the mixture among muffin cups, filling each about three-quarters full.
05 - Bake in the preheated oven for 18 to 22 minutes, or until muffins are puffed and a toothpick inserted into the center emerges clean.
06 - Allow muffins to cool in the pan for 5 minutes. Carefully loosen edges with a knife and transfer to a cooling rack or serve immediately.