Arrange a rack in center of oven; preheat to 425°. In a large bowl, combine beef and pork; season with salt and black pepper. Add onion, egg, garlic, Parmesan, breadcrumbs, and parsley. Using your hands or a rubber spatula, mix until just combined.
Divide mixture into 4 pieces (about 11 oz. each) and form into a ball. Arrange on a baking sheet and drizzle tops with oil.
Bake meatballs until tops and bottoms start to turn brown, about 15 minutes.
In a large Dutch oven or heavy ovenproof pot over medium heat, heat oil. Add onions and garlic and cook, stirring occasionally, until softened, about 7 minutes. Add tomatoes and water and stir to combine. Add basil and oregano; season with salt, black pepper, and red pepper flakes. Bring to a simmer, stirring often, then reduce heat to medium-low and simmer, stirring occasionally, until slightly reduced and flavors have melded, about 10 minutes.
Reduce oven temperature to 325°. Transfer meatballs to marinara and spoon sauce over.
Continue to bake until meatballs are cooked through and an instant-read thermometer inserted into the center registers 165°, about 30 minutes more.
Serve meatballs with pasta. Spoon more sauce over. Top with Parmesan.