Preheat the oven to 350 degrees and lightly grease a 9×13 baking dish.
In a large skillet, brown the ground sausage until cooked through about 8 minutes. Pour the sausage in a bowl lined with paper towels to drain the grease, set aside.
Wipe down the skillet if there is any excess grease and then add the onion back into the skillet. Cook the onion on medium heat for 5 minutes and then add the garlic. Add the sausage back into the pan and top with marina sauce. Stir and let the sauce simmer on low heat for 5 minutes.
While the sauce is simmering in a medium bowl, mix the cottage cheese, parsley, garlic and egg.
If using a knife or mandoline, slice the sweet potatoes in very thin rounds. If using a vegetable sheet cutter, use the thin blade and make sweet potato sheets.
Begin the bottom layer by laying the sweet potato along the bottom of the dish. If using rounds, let them overlap just slightly so the entire bottom of the dish is covered.
Scoop about a cup of the meat sauce over the bottom layer. Next, layer half of the cheese mixture over the sauce. Continue with a layer of sweet potatoes and then repeat with the sauce and cheese mixture.
For the final layer, lay down one last potato layer and then top with the remainder of the sauce. Lastly, sprinkle the mozzarella cheese over the top and garnish with fresh parsley.
Cover the dish with aluminum foil and bake for 40 minutes. Remove the foil and then cook for another 5 minutes.
Remove the lasagna from the oven and let stand for at least 10 minutes before cutting.