Preheat oven to 375° F. and grease a 9x13 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, then 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.

Layer sweet potatoes in bottom of prepared baking dish, slightly overlapping. Spread about ⅓ of ricotta mixture over sweet potatoes then pour ⅓ of meat sauce on top. Repeat to make two more layers. Top lasagna with mozzarella slices and remaining ¼ cup Parmesan.

Cover with aluminum foil and bake until sweet potatoes are almost cooked through, about 45 minutes. Remove foil and bake until sweet potatoes are fork tender, 15 minutes more. Let rest 10 minutes. Garnish with parsley before serving.