Finely slice the onion and chop the red pepper. Cut the tomatoes in half. Shred or thinly slice the cabbage leaves.
Heat the oil in a frying pan and gently sauté the onion and red pepper for 5 minutes, then add the chilli and tomatoes. Put a lid on and cook for another 5 minutes.
Add the balsamic vinegar and cabbage slices, then put the lid back on. After a couple of minutes add a splash of water if it’s too sticky. When the cabbage has wilted, season with salt and pepper and add some balsamic leaves. Set aside.
Meanwhile, whisk 3 eggs in a small bowl with 3 pinches of salt.
Lightly grease and heat a flat, non-stick frying pan and pour in the egg mixture, letting it spread out evenly. Scatter some cheese on top, if used. When the surface is almost set, spoon half of the tomato & cabbage mixture onto one half of the omelette, then add some more cheese, if liked, and turn the other half over it. Cook for another minute or so, then transfer it to a plate.
Make the second omelette with the remaining eggs and vegetable mixture.
Serve warm.