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.
Meanwhile, melt 1 tablespoon butter in large nonstick skillet over medium heat.
Add onion and cook until softened and golden brown, about 8 minutes. Transfer to small 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, 9 to 12 minutes.
Stir in onion, garlic salt, salt, and pepper to taste. Serve.