1/2cupcheesefeta, goat, Gorgonzola, Parmesan, etc.
Start the onions by heating the oil and butter over medium heat. Add the onions and cook, stirring for 15-20 minutes until very soft and golden. When done, stir in the vinegar and keep warm.
Meanwhile, precook the potatoes by placing, single layer, in a microwavable pan and cook for about 3 minutes, stirring part-way through, until almost tender (alternately, parboil until just tender).*
In a large (10-inch) nonstick or well-seasoned cast iron skillet, heat the 2 tablespoons oil over medium heat. Add the potatoes and garlic and cook until browned and tender, about 5 minutes. Spread the potatoes as evenly as possible in the skillet.
Position the broiler rack about 6 inches from the element and preheat the broiler.
In a 4-cup measure, whisk the eggs, salt, pepper, and rosemary. Pour over the potatoes and reduce heat to medium-low.
Using a rubber spatula, lift up the cooked edges and tilt the skillet to allow the uncooked eggs to run underneath. Continue cooking, occasionally lifting the frittata and tilting the skillet until the top is almost set (about 5 minutes).
Place under broiler for about 1-2 minutes or until it's puffed and browning. Sprinkle with cheese and replace under broiler for 1 more minute, or until melted or softened.
Serve topped with the onions (either on top of the whole frittata or on each piece as you serve it).
*Or use leftover roasted potatoes. If you have leftover roasted potatoes, you can skip this step and simply add the leftover potatoes to the skillet, cooking them in the skillet just to warm before adding the eggs.