Crispy Roasted Rosemary Sweet Potatoes

butter, melted 3 Tablespoons olive oil 1/4 teaspoon crushed dried rosemary, or 1/2 tsp. fresh 3 lbs. (3-4 medium) sweet potatoes, peeled and sliced thinly 1 shallot, peeled and sliced thinly Kosher salt and freshly cracked black pepper, to taste