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.
Melt remaining 2 tablespoons butter in now-empty skillet over medium heat. Add potatoes and pack down with spatula. Cook, without moving, until underside of potatoes is brown, 5 to 7 minutes. Turn potatoes, pack down again, and continue to cook until well browned and crisp, 5 to 7 minutes. Reduce heat to medium-low and continue cooking, stirring potatoes every few minutes, until crusty and golden on all sides, 9 to 12 minutes.
Stir in onion, seasonings of your choice and salt and pepper to taste.