Preheat oven to 325F. Heat 1 Tbsp butter or oil in a large, heavy skillet; add the onions, 1/2 tsp salt, and freshly ground pepper to taste. Sauté over medium heat for about 10 min. Add the garlic and continue to cook over medium heat until onions turn deep golden and caramelize.
Beat the eggs in a medium-sized bowl. Stir in the onions along with the hard cheese and sage. In a cast-iron skillet, heat the remaining butter or oil until hot and swirl around the pan. Turn heat low and pour the frittata mixture in (the eggs will sizzle from the heat). Crumble the goat cheese and cook over low heat for 5 min., until the sides begin to set. Transfer to the oven and bake, uncovered, for 20-25 min, until the frittata is golden and firm.
Loosen gently with a spatula and flip onto plate. Cut into wedges and serve warm, with a garden salad and bread.