Preheat the oven to 400°F.
To begin, coat a saute pan with olive oil and bring to medium heat.
Add the onions and a pinch of salt and cook until soft and translucent.
Add the garlic and saute for a couple of minutes. Turn off heat and allow the mixture to cool.
Put the ground beef, sausage, eggs, Parmigiano-Reggiano, parsley, bread crumbs, cooled onion mixture and water in a large bowl. Season with salt and pepper and use your hands to combine. The mixture should be well combined, but try not to overmix.
Test the seasoning of the mixture by making a small meatball and tasting it.
Shape the meatballs. Coat a large, heavy bottomed pot with olive oil and bring to medium heat. Brown the meatballs on all sides. Don't be afraid to let them get some color (it adds flavor).
Pour the tomato sauce over the meatballs and top with mozzarella and Parmigiano Reggiano.
Cover the skillet with a lid or foil and bake for 30 minutes. Then remove the cover and bake for an additional 10 minutes or until the top is golden.