While the sauce is simmering, in a large mixing bowl, combine the two types of ground meat, breadcrumbs, parsley, oregano, egg and olive oil. Season with black pepper (generously) and salt.
Mix the mixture till they are well mixed and slightly sticky. Divide and roll them into 1.5 to 2 inch meatballs. You will get about 16 meatballs.
Preheat the oven to 200 degrees Celsius. Prepare a large baking dish (about 8x8-inch and 3-inch in height would be good)
Heat a frying pan over high heat and drizzle in some olive oil. When the oil is very hot, add in the meatballs and sear them until they are browned all over. Remove them from heat and transfer them to the baking dish. Do cook the meatballs in batches if your pan is small, adding more oil if necessary.
Heat up the tomato sauce. When the sauce is boiling, pour it over the meatballs. Top with mozzarella slices then Parmesan cheese.
Bake in the center rack for 20 minutes until the sauce is bubbling, the cheese has melted and turned into a golden perfection.
While the meatballs are baking, cook the spaghetti. Divide the spaghetti among the serving plates. Use serving plates that can hold a little gravy.
Let the meatballs stand at room temperature for a couple of minutes, then scoop the meatballs with the cheese over the pasta. Spoon plenty of the delicious and flavourful tomato sauce all over - the sauce would have turned watery during baking, becoming a soup base for the spaghetti!