01 - In a mixing bowl, whisk together eggs, milk (if using), salt, and black pepper until well combined.
02 - Heat half the oil or butter in a non-stick skillet over medium heat. Add bell pepper, spinach or kale, and red onion. Sauté for 2 to 3 minutes until just softened. Remove vegetables from skillet and set aside.
03 - Add remaining oil or butter to the skillet. Pour in half of the egg mixture, tilting the pan to distribute evenly. Cook for 1 to 2 minutes until almost set.
04 - Sprinkle half of the sautéed vegetables and, if using, half the shredded cheese over the eggs. Carefully roll the omelette from one end using a spatula. Transfer to a plate.
05 - Repeat the cooking, filling, and rolling process with the remaining egg mixture and filling for the second roll.
06 - Slice each roll into two pieces. Serve warm or at room temperature.