Heat olive oil in a 6-8 inch nonstick skillet over medium heat. Once hot, add the spinach handful by handful and cook 4-5 minutes or until spinach is wilted and hot; transfer to a plate and set aside.
Meanwhile, whisk together the eggs and milk. Return skillet to medium flame and mist with cooking spray, if needed. Once hot, add half the eggs. Cook 1-2 minutes, gently lifting the edges of the cooked egg and tilting the pan to allow additional egg to run underneath; repeat around the edges of the omelet; cover and cook 1-2 minutes. Top half of the egg surface with ½ ounce feta, 2 tablespoons of diced roasted red pepper, and half of the spinach; carefully fold the omelet in half so that the ingredients are layered inside; cook an additional 1-2 minutes or until egg is cooked through. Serve hot with salt and pepper, if desired.