I use small red potatoes. I don't peel potatoes as a rule, you can if you prefer. I just wash them thoroughly and then slice.
2 1/2 tablespoons butter
12 sprigs thyme, strip the leaves from the stems
1 large onion, sliced somewhat thickly ( a tad more than 1/4 inch thick)
1 lb. waxy red potatoes, washed and very thinly sliced
2 tablespoons olive oil
1/2 cup grated gruyère
1/2 cup crumbled feta
1 cup grated mozzarella
sea salt and cracked black pepper
Preheat oven to 400°. Melt the butter in a 9-10 inch ovenproof frying pan (cast iron is great) over medium heat. Add the thyme and onion and cook for 5 minutes. Place the potato, oil, cheeses, salt and pepper in a bowl and toss to combine.
Top the onion with the potato mixture and bake for 45 minutes or until golden brown and cooked through. Turn upside down to serve.