For Salad: Pre-heat oven to 425 degrees. Place sweet potatoes in a single layer on a baking sheet and drizzle with oil, turn to coat and season with cumin, salt and pepper. Roast for 25 to 30 minutes in oven or until tender. Allow to cool. (sweet potatoes can be made a day in advance). To assemble place mixed greens on serving platter, add roasted sweet potatoes, crumbled queso fresco and pepitas. Dress with vinaigrette and serve.