Upside Down Potato Onion Tart 2 1/2 TBS butter 12 sprigs thyme 1 lg onion, sliced 1/4 inch thick 1 lb. potatoes thinly sliced 2 TBS olive oil 1/2 C grated gruyère 1/2 C crumbled feta 1C grated mozzarella salt and pepper Preheat oven to 400°. Melt butter in a 9 in cast iron skillet. Add thyme and onion and cook 5 min. Toss the potato, oil, cheeses, salt and pepper and add on top of onion mix and bake for 45 min. Turn upside down to serve.