How to make the meatballs: In a medium bowl, whisk together the egg with Parmesan until incorporated. Set aside.
In a large bowl, combine ground meat, bread crumbs, onion, garlic, parsley, salt, and the Parmesan/egg mixture. Mix everything together until just incorporated. Take care not to overwork the meat! Using an ice cream scoop, two spoons, or your hands, portion out the meatballs. This recipe makes about 20 medium sized meatballs, but you can use it to make larger or smaller portions (cooking times will vary). In a large skillet, add olive oil and butter. Heat over medium heat until the butter is melted and sizzling. Add the meatballs to the hot oil and butter. Cook over medium heat on one side until golden brown (about 1 to 2 minutes). Flip the meatballs and cook on the other side for another minute or so, or until golden brown.
Transfer the meatballs to a plate and set aside to make the marinara sauce.
How to make the marinara sauce: Working with the same skillet the meatballs were cooked in, add a tablespoon of olive oil if needed. Add the onion, garlic, and a pinch of salt to the skillet. Cook over medium-low heat until the onion softens and turns translucent (about 2 to 3 minutes).
Add oregano and basil, and cook for another minute to toast the herbs.
Add tomato sauce and water to the skillet. Bring the sauce to a boil over high heat, then reduce heat to low to maintain a simmer. Add meatballs back into the marinara sauce and simmer over low heat for 15 to 20 minutes, or until the meatballs are cooked through. Flip meatballs halfway through cooking time.