Heat 1 teaspoon of the oil in a 26 cm (10-inch) non-stick frying pan over medium heat. Pour in one-quarter of the eggs and swirl to make an even layer.
Use a spatula to draw the egg into the centre of the pan from the side as it begins to set. After about 1 minute the egg will be set around the side and underneath but moist on top. Sprinkle with the spinach and feta. Fold one-third of the omelette towards the centre, then fold over again to enclose the filling.
Slide the omelette onto a plate and serve immediately with the tomatoes, seasoned with freshly ground black pepper. Repeat to make three more omelettes.
If you prefer, cook the tomatoes in a little oil in a frying pan over medium heat for about 5 minutes, until the skins just start to split. Keep them warm while you cook the omelettes.