1. Preheat oven to 400ºF. Place potatoes on a large baking sheet and toss with 3 Tbsp. olive oil and salt. Roast until browned, 30 to 40 minutes, turning over once halfway through baking time.
2. Combine garlic and remaining Tbsp. oil in a small bowl. Brush sweet potatoes with garlic and oil and roast until garlic is cooked, 2 to 3 minutes.