Preheat the oven to 375°F. Cover a baking sheet with parchment paper.
Rinse and pat dry the potatoes. Toss the potatoes with 2 Tbsp olive oil. Spread in a single layer on the baking sheet. Season with kosher salt..
Bake for 40-50 minutes or until fork tender. Cool completely and chill.
To grill: Mix together 3 Tbsp olive oil, chopped fresh rosemary, minced garlic, garlic salt and black pepper.
Oil the grates of a gas grill and heat to medium-high.
Use a spatula and gently press each potato to "smash." to around 3/4 inch thickness.
Brush the potatoes with the seasoned olive oil.on all sides.
Grill for 3-4 minutes on each side until golden and heated through.
Carefully remove to a platter.
Serve with a sprinkle of grated Parmesan cheese, if desired.