Preheat the oven to 400º F.
Make Sour Cream and Onion Dip by combining sour cream, milk, green onions, and onion powder in a small bowl. Set aside in the refrigerator.
Mix parmesan cheese, garlic powder, onion powder, smoked paprika, salt, pepper and cayenne (if using) together in a small bowl.
Cut potatoes in half and set aside.
Pour the olive in a 9 x 13 inch baking dish, tilting the pan to coat the bottom completely. Using a spoon, sprinkle the parmesan mixture evenly over the olive oil. Don’t stir.
Place the potatoes (cut side down) on the parmesan, pressing firmly.
Drizzle olive oil over the potatoes and sprinkle with salt, if desired.
Bake for 36-40 minutes, or until potatoes are soft and the parmesan is a dark golden brown.
Let potatoes rest for 5 minutes to allow a nice crust to form. Use a spatula or fork to break the potatoes apart and flip them over.
Serve from the baking dish, or remove to a platter.
Garnish with sliced green onions and serve with Sour Cream and Onion Dip.