Tilapia with Purple Potato Crust and Chive Rosemary Oil

INGREDIENTS

• 2 tablespoons butter
• 1/2 teaspoon salt
• 1/2 cup extra-virgin olive oil
• 1/4 teaspoon freshly ground black pepper
• 2 tablespoons olive oil
• 8 to 12 ounces purple potatoes, peeled and very thinly sliced
• 4 (4- to 6-ounce) tilapia fillets
• 1/3 cup minced fresh chives
• 1 tablespoon fresh minced rosemary leaves, plus 1 tablespoon