Preheat your oven to 350°F (175°C). In a large mixing bowl, combine the egg, chopped parsley, tomato paste, oregano, salt, pepper, and minced garlic. Mix in the ground meats, breadcrumbs, and grated Parmigiano-Reggiano, ensuring it's well combined without overworking. Roll the mixture into golf ball-sized meatballs, about 1-inch in diameter, and place them on an ungreased baking sheet. Bake for 20 minutes, flipping halfway, until they're golden brown and cooked through.

While the meatballs are baking, heat a large skillet over medium heat and add three tablespoons of butter. Once melted, sauté the diced onion for about 5 minutes, stirring occasionally, until it becomes translucent and fragrant. Add in the minced garlic, cooking for another minute until aromatic. Pour in the marinara sauce and let it simmer gently on low heat while seasoning with a sprinkle of sugar, salt, and pepper.

After simmering the marinara sauce for about 5 minutes, slowly stir in the heavy cream, allowing it to gently simmer for an additional 2-3 minutes.

In a large pot, bring salted water to a boil. Break the spaghetti in half and add it to the boiling water. Cook according to package instructions, usually around 8-10 minutes, until al dente. Just before draining, reserve a cup of the pasta water.

Once the spaghetti is cooked, drain it well and add it directly to the creamy marinara sauce in the skillet. Toss the pasta in the sauce for about 5 minutes over medium heat, allowing it to absorb the creamy goodness. Gently fold in the baked meatballs, ensuring they’re coated in that delightful sauce.