Preheat the oven to 200℃ (180℃ fan)/400°F/gas mark 6. Heat the oil in a frying pan and fry the onion for about 5 minutes until softened. Add the spinach and cook until wilted. remove from the heat. Tip the spinach into a well oiled baking tin about 30x 20cm ( 12 x 8 in). Arrange the cherry tomatoes in the tin then add the mozzarella In a bowl, beat the eggs together with fork. Season with salt and pepper and beat again. Pour the egg into the tin and bake for about 20 minutes until pale golden and set. Serve hot or cold.
Nutrition information is approximate and is meant as a guideline only. (It does not include seasoning with salt and pepper.)