Meanwhile, whisk together eggs, mustard, vinegar, salt, and pepper. Coat a large oven-proof skillet with cooking spray. Add oil and onion, sautéing until tender. Add kale and cook until just wilted, about 2 minutes. Add sweet potato and toss. Spread vegetables around the bottom of the skillet. Carefully pour the egg mixture over the vegetables. Cook for 3 minutes or until bottom of frittata is just set. Transfer skillet to oven and cook for about 10 minutes or until the entire frittata is set. Remove from oven, sprinkle with goat cheese and enjoy!