01 - Preheat oven to 375°F. Lightly grease a 9×13-inch baking dish with nonstick spray or butter.
02 - Place the diced sweet potatoes in a medium pot and cover with 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 the chopped onion and minced garlic, sautéing until softened and fragrant, about 3 minutes.
04 - Add the ground beef to the skillet, breaking it apart with a wooden spoon. Cook until fully browned and no pink remains, 5 to 7 minutes. Drain off any excess fat.
05 - Stir in the drained diced tomatoes, dried oregano, paprika, salt, and pepper. Cook for 2 to 3 minutes to allow the flavors to meld. If using spinach, fold it in and cook until just wilted. Remove from heat.
06 - Spread 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 evenly over the sweet potato layer, spreading it to the edges.
08 - Dollop the cottage cheese over the beef layer and gently spread it into an even coating using the back of a spoon.
09 - Arrange the remaining sweet potatoes over the cottage cheese. Sprinkle the shredded mozzarella and grated Parmesan evenly across the top.
10 - Bake uncovered for 25 to 30 minutes, until the cheese is melted, bubbly, and lightly golden on top.
11 - Remove from the oven and let the casserole rest for 5 minutes before slicing and serving. Garnish with fresh parsley or chives if desired.