Preheat oven to 400F.
In a medium microwave safe bowl, add sweet potato and ½ cup water. Cover and cook (on high power) until tender (approx. 5 min.) Drain.
While sweet potato is cooking, whisk together in a medium bowl: eggs, mustard, vinegar and salt and pepper to taste.
Spray a large ovenproof nonstick skillet with cooking spray. Over medium-high heat add oil. When oil is hot, add onion and cook until onion begins to soften (approx. 3 min.) Add kale and continue to cook until kale wilts but is still bright green. Add sweet potatoes and stir to blend all ingredients.
Pour egg mixture carefully over vegetables. Cook until the bottom of the frittata is set (approx. 3 min.).
Place the skillet in the oven and bake until the frittata is set in the center (approx. 10 min)
Loosen the frittata from the pan with a heat-proof spatula, and carefully slide the frittata onto a plate or platter. Garnish with cheese and let set stand for a few minutes until cheese softens a bit. Cut into 4 wedges and serve.