Tumpukan Ubi Jalar Panggang

Bahan-bahan (12 porsi)

• 1.2 kg / 2.4 lb sweet potato (, unpeeled weight, about 5 - 6.5 cm / 2 - 2.7" wide)
• 50 g / 3 tbsp unsalted butter (, melted)
• 1 1/2 tbsp olive oil
• 2 tbsp finely chopped fresh rosemary ((or 1 tbsp dried, or other herbs of choice))
• 1 1/4 tsp salt
• 1/2 tsp black pepper