Preheat oven to 375° and grease a 9”-x-13” baking pan with cooking spray. In a large skillet over medium heat, heat oil. Add onion and cook until soft, 5 minutes then add garlic and cook until fragrant, 1 minute more. Add sausage and cook, breaking up meat with a wooden spoon, until no longer pink, about 6 minutes. Season with salt and pepper.
Pour in marinara sauce and bring to a simmer. Cook until sauce thickens slightly, 5 minutes. Add spinach and cook until spinach is wilted. Remove from heat.
In a large bowl, stir together ricotta, egg, ½ cup Parmesan, and oregano. Season with salt and pepper.
Spread a thin layer of marinara mixture evenly across the bottom of prepared dish. Layer sweet potatoes on top of sauce, slightly overlapping. Spread about ⅓ of ricotta mixture over sweet potatoes then pour ⅓ of remaining meat sauce on top. Repeat to make two more layers. Top lasagna with mozzarella and remaining ¼ cup Parmesan.
Cover with aluminum foil and bake until sweet potatoes are almost cooked through, about 45 minutes, then remove foil and bake until sweet potatoes are fork tender and cheese is lightly golden, 15 minutes more.
Let rest 10 minutes, then garnish with parsley before serving.
