Heat 2 tsp oil in a large non-stick frying pan over medium heat. Add the onion and garlic; sauté 5 minutes. Add green capsicum and tomato; sauté 3 minutes. Add roasted capsicum, potato and salt. Cover and cook, stirring frequently, until the vegetables are tender, about 10 minutes. Add a little water if the vegetables begin to stick.
Coat a large non-stick frying pan with non-stick cooking spray. Heat over medium heat. Swirl in remaining oil. Add the eggs; cook, without stirring, until they begin to thicken slightly around the edge of the pan, about 1 minute. Run a thin spatula around the edge of the pan, lifting mixture so uncooked egg flows under the cooked portion. Cook until centre is still moist but no longer runny, about 3 minutes. Slide the omelette onto a plate. Top with vegetable mixture.