Heat oil in a large skillet over medium-low heat. Once hot, cover bottom of pan with half the potatoes. Season with salt and pepper. Dot with half the butter and repeat so all the potatoes and butter are used. Cook until potatoes begin to brown on the bottom, 10 to 15 minutes. Add onions and season with salt and pepper. Cook, stirring, until onions and potatoes are cooked through, 8 to 10 minutes. Add garlic and cook until fragrant, about 1 minute. Toss together.
Using a spatula, gently lift potatoes out of pan and place on a serving platter. Garnish with parsley.
Serves 4 to 6