Upside Down Potato & Onion Tart

INGREDIENTS

• 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 gruyere cheese
• 1/2 cup crumbled feta cheese
• 1 cup grated mozzarella cheese
• sea salt and black pepper