Heat a little olive oil in a frying pan over medium heat. Add the chopped onion and sliced mushroom. Cook until softened and lightly golden.
Beat the eggs in a bowl with a fork. Season with salt and pepper.
Add a small swirl of olive oil to the pan to ensure the omelette doesn’t stick. Pour in the beaten eggs, swirling the pan so the mixture covers the base evenly. There should be no gaps.
Scatter over the chopped sundried tomato and spinach. Crumble a generous amount of feta over the top.
Place a lid on the pan and cook for around 3 minutes, or until the egg is set but still soft. Gently shake the pan a few times while cooking to make sure the omelette isn’t sticking.
Fold the omelette in half and slide onto a plate.
