Preheat your oven to 350°F (175°C). In a large mixing bowl, combine the egg, chopped parsley, tomato paste, oregano, salt, pepper, minced garlic, and water. Gently add ground beef, ground pork, bread crumbs, and Parmigiano-Reggiano cheese. Mix until just combined without overworking the meat. Roll the mixture into golf ball-sized meatballs and place them on a baking sheet lined with parchment paper. Bake for 20 minutes, flipping halfway through, until they are browned.

In a large skillet, melt butter over medium heat. Add chopped yellow onion, sautéing for about 5 minutes or until softened and translucent. Stir in minced garlic and cook for an additional minute until fragrant. Next, pour in the high-quality marinara sauce; bring it to a gentle simmer. Incorporate sugar and season with salt and pepper to taste, allowing the flavors to meld, which should take around 2-3 minutes.

Once the sauce has simmered, reduce the heat slightly and stir in the heavy cream. Allow the sauce to simmer for about 2-3 minutes longer until it thickens slightly and becomes creamy. This step is crucial for achieving that luscious texture that pairs perfectly with the meatballs.

While the sauce simmers, bring a large pot of salted water to a boil and cook the spaghetti according to the package instructions, usually around 8-10 minutes, until al dente. Before draining, reserve about a cup of pasta water, which can help adjust the sauce's consistency later. Drain the spaghetti and return it to the pot, keeping it warm.

Add the freshly cooked spaghetti to the creamy sauce in the skillet, stirring gently for about 5 minutes to help the pasta absorb the rich flavors. Carefully add the baked meatballs to the skillet, tossing them in the sauce to coat evenly. Serve the Creamy Spaghetti and Meatballs hot, garnished with extra Parmigiano-Reggiano cheese.