Cut the mushrooms and courgette into bite-sized pieces. Cut the bell pepper into thin slices. Finely chop the spring onions.
Hat the vegetable oil and gently stir-fry the mushroom, courgette and bell pepper for a couple of minutes until soft but still have a bite. Add the soy sauce and spring onions, then reduce the sauce.
In a small bowl, lightly whisk the eggs with a few pinches of sesame seeds and a drop of sesame oil.
Pour the egg mixture over the vegetables, letting it spread out evenly. Scatter the chilli slices on the top, if used, then season with black pepper.
Cook for a couple of minutes over medium heat, then put a lid on and cook for about 10 minutes, until the surface is no longer jelly-like. Alternatively, put the frying pan in a preheated oven/grill to brown the top.
Transfer the omelette to a plate, then cut it into wedges. Serve with some sesame seeds scattered on top.