Double Potato Gratin

Ingredients

• 3 lb (1.5 kg) Yukon Gold or baking potatoes (about 6 large)
• 1 lb (500 g) sweet potatoes (about 2)
• 1 tbsp (15 mL) butter, divided
• ¼ cup (50 mL) olive oil
• 2 tsp (10 mL) salt
• 1 tsp (5 mL) pepper
• 1 tbsp (15 mL) chopped fresh rosemary or thyme (or 1/2 tsp (2 mL) dried)