Place sliced potatoes in a pot of cold, salted water and bring to a boil. Cook for 5 minutes, then drain and pat dry.
In a large skillet over medium heat, melt 2 tbsp butter with 1 tbsp olive oil. Add onions and cook until caramelized, about 10 minutes.
Remove onions and set aside. Add remaining butter and olive oil to the skillet, then add potatoes in a single layer.
Cook potatoes undisturbed for 4-5 minutes until golden brown. Flip and cook the other side.
Return the onions to the skillet, stir in garlic, and season with salt and pepper. Cook for another 2 minutes.
Garnish with fresh parsley and serve warm.