Melt the butter in a frying pan over a medium heat. Add the garlic, mushrooms and spring onions and cook and stir until the mushrooms are golden brown; transfer to a mixing bowl. Add the remaining ingredients, except the grated cheese and mix well. Pour into a medium ovenproof baking dish (choose one so as to result in a frittata that is about 4 to 5cm deep). Sprinkle the top of the frittata with grated cheese.
Bake in the preheated oven until a skewer inserted into the centre comes out clean, about 18 to 25 minutes. Remove from the oven and serve warm, or allow to cool before serving.