Heat 2 tablespoons oil in a large Dutch oven over medium-high heat. Add sweet potato; cook, stirring occasionally, until lightly browned and beginning to soften, 6 to 8 minutes. Add leeks and carrots; cook, stirring occasionally, until softened, 3 to 4 minutes. Stir in garlic, tomato paste, cumin and the remaining 1 tablespoon oil; cook, stirring constantly, until fragrant and the tomato paste has darkened, about 1 minute.