Mushroom Spinach Fritatta, 271 calories Fast Day Lunch: Mushroom And Spinach Frittata Recipe Per Serving: 271 cals Ingredients: Half an onion, 1 tsp olive oil mushrooms, 20g spinach, 2 eggs, beaten - Method: Slice the onion and fry it in the olive oil. Add the chopped mushrooms and cook until tender. Add the spinach and cook for 2 minutes. Pour over 2 beaten eggs and cook for 5 minutes. Then finish under the grill.