Sunnys Crispy Smashed Rosemary Garlic Fingerling Potatoes

Name: Sunny's Crispy Smashed Rosemary Garlic Fingerling Potatoes
Ingredients: One 1 1/2-pound bag steamable fingerling potatoes; 3 tablespoons olive oil; 3 tablespoons salted butter, plus more if needed; Two 6-inch stalks fresh rosemary; Kosher salt and freshly ground black pepper; 1/4 cup finely chopped fresh parsley; 2 cloves garlic, minced or grated on a rasp grater