Steam, microwave or boil potatoes just until fork tender. Drain, set aside to cool.
To prepare: Preheat oven to 425°F. Brush a rimmed sheet pan with 1 Tbsp olive oil.
In a small bowl, mix together Parmesan, garlic, salt, onion powder and pepper.
Arrange potatoes on baking sheet. Use a potato masher or fork to smash to about ½ inch thickness. Mix together remaining olive oil and butter. Brush over potatoes.
Sprinkle evenly with Parmesan-seasoning blend.
Bake for 25-30 minutes until edges are golden. Garnish with parsley and serve immediately.