Begin by precooking the potatoes. Arrange them in a microwave safe bowl, top with 1 tablespoon of butter and cover tightly with plastic wrap. Microwave on high until the edges of the potatoes begin to soften, 5-7 minutes, shaking the bowl (without removing plastic wrap) half-way through cooking.
Meanwhile, melt 1 tablespoon butter in a large nonstick skillet over medium heat. Add the onion and cook until softened and golden brown, about 8 minutes. Transfer to a small bowl.
In the same skillet, melt the remaining 2 tablespoons of butter over medium heat. Add the potatoes and pack them down with a spatula. Cook, without moving, until one side is brown, 5-7 minutes.
Turn the potatoes, pack down again and continue to cook until well browned, another 5-7 minutes.
Reduce heat to medium low and continue cooking the potatoes, stirring every few minutes, until crusty and brown, 9-12 minutes.
Stir in the reserved onions, garlic salt and salt. Pepper to taste. Serve at once.
NB - remember to turn down the heat for the last 9-12 minutes of cooking or the fries will burn.
Cook's note - the best potatoes to use - medium starch or waxy varieties - Yukon Gold or Reds. High starch varieties such as russets and Idahos will break down leaving nothing but a greasy mash of spuds in your skillet.