1. Preheat the oven to 220°C (425°F).

2. Peel and dice the onions into small pieces. Heat oil in a pan and fry the onions until they're soft and translucent.

3. Add the ground beef to the pan, breaking it up into small pieces as it cooks, until it's nicely browned. Add the taco seasoning and cook for 1 minute, stirring constantly.

4. Stir in the arrabbiata sauce and simmer for 5 minutes, adding salt and pepper to taste as needed.

5. Cut the meatballs into slices, roughly the same size as your oven dish (about 15x20 cm or 6x8 inches). Set aside any leftover meatballs for another meal.

6. Spoon the beef mixture into the oven dish, followed by a generous layer of grated cheese and a layer of sliced meatballs on top. Brush the meatballs with beaten egg for a nice glaze.

7. Use a fork to prick a few small holes in each meatball, allowing steam to escape while they cook.

8. Bake the casserole in the preheated oven for about 15 minutes, or until the cheese is melted and bubbly and the meatballs are golden brown.