Melt the butter in a non-stick pan, add the mushrooms and garlic. Season with salt and pepper and sauté for 6-8 mins until golden and tender.
Meanwhile beat the eggs and nutmeg in a large bowl and season well. Pour into the pan and stir well, cook over a gentle heat for a minute. Shake the pan to level the frittata, cover with a baking sheet or a pan lid and cook over a gentle heat for 6-8 mins or until the egg is almost set.
Scatter over the remaining cheese and cook under a hot grill for a further 4-5 mins until the top is golden and set.
Leave to cool in the pan for pan for 5 mins before cutting into wedges and serving warm.
Notes
You can eat this frittata cold if you prefer - it makes a great picnic dish.