Preheat the oven to 350°F. Lightly grease a 9x13-inch baking dish.
Heat the olive oil in a large, deep skillet over medium heat. Add the onion and sauté for 3–4 minutes until translucent. Add the crushed garlic and cook another 30 seconds, just until fragrant.
Stir in the tomato paste and cook for 1 minute to deepen its flavor and caramelize slightly.
Add the ground pork sausage to the skillet and cook for 8–10 minutes, breaking it apart with a spoon until browned and cooked through. Drain off excess fat if needed.
Add the crushed tomatoes, Italian herbs, salt, and pepper. Stir well, then reduce heat to low and let the sauce simmer for 10–12 minutes, stirring occasionally, until thickened.
While the sauce simmers, bring a large pot of salted water to a boil. Add the spaghetti and cook to al dente, about 1–2 minutes less than the package instructions. Drain but do not rinse—the residual starch helps the sauce cling to the pasta.
Remove the skillet from the heat and stir in the cream cheese until fully melted and blended into the sauce. This creates a creamy, cohesive texture. If the sauce seems overly thick, stir in 1/4 cup of the starchy pasta water.
Add the drained spaghetti to the skillet and stir or toss until the noodles are evenly coated. Adjust seasoning as needed.
Transfer half the coated pasta to the prepared baking dish and sprinkle with half the mozzarella and Parmesan. Add the remaining pasta and top with the rest of the cheese.
Bake uncovered for 15–20 minutes, until the cheese is melted and bubbling with light golden spots.
Remove from the oven and rest for 5 minutes. Sprinkle with chopped fresh parsley before serving.