01 - Preheat oven to 375°F. Lightly grease a 9x13-inch baking dish and set aside.
02 - Place diced sweet potatoes in a medium pot of salted water. Bring to a boil and cook until just tender, 8 to 10 minutes. Drain thoroughly and set aside.
03 - Heat olive oil in a large skillet over medium heat. Add chopped onion and minced garlic, sautéing until softened and fragrant, about 3 minutes.
04 - Add lean ground beef to the skillet. Cook, breaking it apart with a spoon, until evenly browned throughout, 5 to 7 minutes. Drain any excess fat if necessary.
05 - Stir in drained diced tomatoes, dried oregano, paprika, salt, and pepper. Continue cooking for 2 to 3 minutes. Fold in baby spinach if using, and cook until just wilted. Remove from heat.
06 - Arrange half of the parboiled sweet potatoes in an even layer across the bottom of the prepared baking dish.
07 - Spoon the seasoned ground beef mixture over the sweet potato layer, spreading it into an even covering.
08 - Dollop cottage cheese over the beef layer and spread gently into an even distribution using the back of a spoon or spatula.
09 - Layer the remaining sweet potatoes over the cottage cheese. Sprinkle shredded mozzarella and grated Parmesan evenly across the top.
10 - Bake uncovered for 25 to 30 minutes, until the cheese is bubbly and golden brown on top.
11 - Remove from the oven and let the casserole rest for 5 minutes to set before slicing and serving.