Preheat your oven to 375°F (190°C). Lightly grease a 9×13-inch glass casserole dish with 1 tablespoon butter or cooking spray. Set aside.
Caramelize the onions: In a large skillet, melt 2 tablespoons butter over medium heat. Add the sliced onions and 1/2 teaspoon of the salt. Cook, stirring occasionally, for 20–25 minutes, lowering the heat if they start to brown too quickly. If they seem dry or start to scorch, add 1 tablespoon olive oil and a splash of water. Sprinkle in the optional 1 teaspoon
