Add the egg whites to a large mixing bowl and whisk with an electric whisk until fluffy and increased in size by 50%. This takes about 1 minute. Then, whisk in the parmesan, Dijon mustard, salt and pepper.
Heat 2 teaspoons of oil in a non-stick frying pan. Fry the peppers for 1 - 2 minutes on medium heat until just soft, then the spring onions and spinach until wilted, about 1 minute.
Pour the cooked veggies into egg white mixture and stir to combine.
Add 1 teaspoon of oil to the pan to prevent sticking. Then, pour in the veggie egg mix and swirl the pan around to coat the bottom. Reduce the heat to low.
Cook the omelette on low heat or until just set on top or to your liking. Mine took 7 - 8 minutes without moving the egg mixture during cooking. Option to flip and cook for 30 more seconds to brown the other side. Fold into a half-moon shape and serve.
