Preheat the oven to 375 degrees.
In a large bowl combine the potatoes, onion, and olive oil. Place in an 8 x 8 baking dish and bake for 30 minutes, stirring once during the baking time.
Remove from the oven and add the scallions, garlic cloves, Ranch dressing, sour cream, and cheddar cheese. Mix well to combine.
Sprinkle with salt and pepper and mix again.
Bake for another 15 minutes then remove from oven and stir.
Increase the temperature of the oven to 425 degrees. Sprinkle with remaining cheddar cheese and bake for 15 additional minutes.
Remove from oven and sprinkle with remaining scallions. Serve hot.