Steam the sweet potatoes for 5 minutes, until just tender. Remove from the heat and drain on paper towels. In a medium, nonstick skillet, heat 1 tablespoon olive oil over medium-high heat. Add the sweet potatoes and cook, shaking the pan and moving the pieces around often, until evenly browned on all sides, about 10 minutes. Remove from the heat and drain on paper towels. Whisk together the lime juice, vinegar, Dijon mustard, garlic, salt pepper, olive oil and buttermilk. Place the salad greens in a salad bowl and top with the cheese. Toss with the dressing. Sprinkle on the sweet potato croutons and serve.