Mushroom and potato frittata

Heat a little of the oil and saut&eacute the mushrooms and onion in a pan with an ovenproof handle until golden brown.
Remove from the pan and wipe it.
Heat the remaining oil in the pan and arrange a third of the potato slices in the bottom of the pan so they overlap slightly.
Season well with thyme, salt and pepper.
Spoon half the mushroom mixture on top.
Repeat with another third of the potato slices, season and spoon the remaining mushroom mixture on top.
End with a layer of potatoes.
Whisk together the eggs and milk and carefully pour over the potato layers.
Cover the pan and cook slowly until the egg mixture has set, about 20 minutes.
Place under a very hot oven grill until golden brown. Slice and serve hot.