1 tbl. olive oil
½ tsp salt
2-3 cloves garlic, minced
1/3 c. Parmesan cheese
2 tbl. butter
6 tbl. balsamic vinegar
1-2 onions sliced thin
1/8 tsp. black pepper
8 eggs
1 tbl. sage
3 oz. feta cheese, crumbled

Heat oil and cook onions, salt, and pepper until onions just start to brown, 3-5 minutes. Reduce heat to low. Continue to cook, stirring occasionally for 30 minutes. Add the garlic; cook until brown and soft, about 10 minutes. Transfer to a bowl and set aside.

Beat eggs in a large bowl. Stir in the cooked onion mixture, Parmesan, and sage.

Melt butter in pan. Pour in the egg mixture and crumble the feta cheese on top. Cook over low heat until the frittata firms up around the edges and moves easily in the pan when you give it a good shake, 8-10 minutes.

While the eggs are cooking, heat the vinegar to a rolling boil. Boil until it reduces by half.

Transfer the frittata to the broiler and broil until it starts to firm up on top and is lightly golden, 30-90 seconds. Be sure to check every 30 seconds. If the egg won't cook to your liking or you don't have a broiler, bake in the oven instead. Slide frittata onto a platter, brush top with vinegar, cut into wedges and serve.