Preheat the oven to 375°.
Place the sweet potato in the spiralizer and cut into "noodles". Set aside.
In a large skillet heat the olive oil over medium high heat. Add the onions and saute 2-3 minutes until softened. Add the sausage and cook 3-4 minutes until heated through and very fragrant. Toss the sweet potato with the onions and sausage. Adjust heat to medium or medium low, so it doesn't burn and cover with a tight fitting lid. Cook potatoes until just tender, about 5-8 minutes. Remove from heat and set aside with lid on the pan.
Break 4 eggs into a bowl and whisk to combine.
Spray a medium skillet liberally with cooking spray and heat over medium high heat. Add the eggs to the skillet and cook for 1 1/2 minutes until bottom of the eggs has set. Top the sweet potato mixture evenly over the eggs and gently press them into the runny eggs.
Whisk the remaining two eggs and pour over the sweet potatoes. Sprinkle the frittata with cheese. Cook the frittata on the stove for 3-4 minutes, then transfer to a hot oven. Cook for 5 minutes or until eggs are set, then turn the oven to broil. Broil for 2-3 minutes until the cheese has melted and starts to brown.