Preheat oven to 400°. Roast sweet potatoes on a small rimmed baking sheet until skins are browned and potatoes are tender all the way through, about 45 minutes. Remove from oven and, using a heavy spatula or small pot lid, smash potatoes, then drizzle with 1 Tbsp. oil; season with salt. Continue to roast until flesh is lightly browned, 12–15 minutes longer.