Melt butter with oil in a non-stick, 25cm frying pan over medium-low heat. Add the leeks and cook for five minutes until soft but not browned. Add the mushrooms and cook for another 4-5 minutes until mushrooms have softened.
Meanwhile, whisk the eggs, crème fraiche and parsley in a large bowl, mix in half the cheese. Season with salt and pepper.
Add the leeks and mushrooms to the egg mixture and stir. Wipe out the pan and add a little more oil and butter and return to a low heat. Add the egg mixture to the pan.
Cook for about 10 minutes until nearly set. Sprinkle over the remaining cheese and place under a medium grill, and cook for about another 10 minutes until the top is set and golden.
Allow the frittata to cool for a couple of minutes then slide the out of the pan onto a board.
Slice into wedges and serve with a green salad and crusty bread.