Make the sauce: In a large saucepan or Dutch oven heat the oil over medium heat and add the sausage, onion and garlic. Break apart the sausage as it cooks and brown on all sides.
Add the tomatoes, Italian seasoning, 1 teaspoon salt and ¼ teaspoon black pepper. Bring to a simmer and cook, stirring often about 25 minutes. Skim the fat off as it rises to the top and discard.
Taste and season as needed. Remove from heat, mix in the parsley and set aside 1 cup of the sauce.
Preheat the oven to 375 degrees F. and place a baking sheet on the bottom rack of the oven to catch any drips. Butter a 9x9-inch square baking or casserole pan with high sides.
Make the lasagna: In a medium bowl mix the sweet potatoes, ricotta, egg, parsley, ½ cup of the mozzarella and the Parmesan together with a couple dashes each of salt and pepper.
Cover the bottom of the pan with a thin layer of the sauce then cover with noodles, breaking them apart to fit if needed.
Spoon half the sweet potato mixture over the noodles and spread out evenly with the spoon. Top with another layer of lasagna noodles.
Layer ½ of the sauce over that topped with ⅓ of the mozzarella. Top with noodles.
Repeat the layers. Top with the remaining sweet potato mixture and evenly spread out.
Top with another layer of noodles, the remaining ½ of the sauce, ⅓ of the cheese and another layer of noodles.
Top with the sauce you set aside earlier and sprinkle with the remaining mozzarella.
Bake about 35 minutes then tent the top with foil and bake another 15 - 20 minutes.
Let the lasagna set 5 - 10 minutes before slicing. Cut into squares to serve.