In a 12-inch skillet over medium-low heat, melt 1 tablespoon of butter with the olive oil. Add the onions and cook for 8–10 minutes, stirring occasionally, until the onions begin to soften.
Place the potatoes and 1 tablespoon of butter in a microwave-safe bowl. Microwave for 6–8 minutes, stirring about every 2 minutes, until the potatoes begin to soften.
Remove the onions from the skillet and place them in a small bowl.
Add the remaining 2 tablespoons of butter to the skillet and increase the heat to medium.
Pour the potatoes into the skillet. Use a spatula to spread them into an even layer and gently press them down. Allow the potatoes to cook undisturbed for 5–7 minutes.
Use the spatula to flip the potatoes over. Press them down again and continue to cook undisturbed for another 5–7 minutes.
Reduce the heat to medium-low and cook for an additional 9–12 minutes, stirring occasionally.
Add the cooked onions, garlic salt, salt, and black pepper. Gently stir to incorporate.
Serve warm.