Arrange potatoes in large microwave-safe bowl, top with 1 tablespoon butter, and cover tightly with plastic wrap. Microwave on high until edges of potatoes begin to soften, 5 to 7 minutes, shaking bowl (without removing plastic) to redistribute potatoes halfway through cooking.
Melt 1 tablespoon butter in large cast iron skillet over medium heat. Add onion and cook until softened and golden brown, about 6 minutes. Transfer to a bowl.
Stir in onion, seasonings of your choice and salt and pepper to taste.