Set the oven to 400°F so it reaches the proper temperature before the assembled casserole is ready to bake.

Warm the avocado oil in a large Dutch oven or heavy-bottomed skillet over medium-high heat. Add the chopped yellow onion and cook for approximately 3–5 minutes, or until it begins to soften.

Stir in the minced garlic and ground beef. Break the meat into small pieces with a wooden spoon or spatula and continue cooking for several minutes, until the beef starts to brown thoroughly.

Add the diced sweet potatoes to the browned beef mixture. Season with the Italian seasoning, sea salt, and ground black pepper, then stir until the ingredients and seasonings are evenly distributed.

Cover the cooking vessel and continue cooking for 5–10 minutes, stirring periodically, until the sweet potato pieces begin to become tender.

Remove the lid and add the baby spinach to the beef and sweet potato mixture.

Cover once more and cook for approximately 1–2 minutes, just long enough for the spinach leaves to wilt and blend into the filling.

Transfer the cooked beef, sweet potato, and spinach mixture to a large casserole dish. Arrange the filling in an even layer to promote uniform baking.

Scatter 1½–2 cups of shredded cheese evenly across the surface.

Place the casserole on the center rack of the preheated oven. Bake for 15–25 minutes, or until the cheese has fully melted and reached the desired degree of browning.

For a deeper golden finish, the casserole may be placed briefly under the broiler during the final couple of minutes. Watch it closely to prevent the cheese from burning.

Remove the finished casserole from the oven and allow it to rest for several minutes before portioning into bowls and serving warm.