Sweet Potato Lasagna

Name: Sweet Potato Lasagna
Prep time: 20 min
Total time: 20 min
Cuisine: American
Ingredients: Cooking spray; 3 large sweet potatoes, peeled and sliced 1/8"-thick (about 2 lb.); 2 tbsp. extra-virgin olive oil; 1 large onion, chopped; 2 cloves garlic, minced; 1 lb. sweet Italian sausage, casings removed; Kosher salt; Freshly ground black pepper; 2 c. marinara; 1 (5-oz.) package baby spinach; 16 oz. ricotta; 1 large egg, beaten; 3/4 c. freshly grated Parmesan, divided; 1 tsp. dried oregano; 1 1/2 c. shredded mozzarella; Freshly chopped parsley, for serving