Boil the spaghetti in salted water following the package instructions until it reaches an al dente texture. Drain thoroughly and toss with a splash of olive oil to keep the strands separate. Set aside.

In a large skillet over medium heat, sauté the ground meat together with the diced onion until fully browned and cooked through.

Incorporate the minced garlic, Italian seasoning, and desired amount of salt and pepper into the skillet. Stir in the jarred marinara sauce and let the mixture simmer gently for 5 to 7 minutes to develop flavor.

In a mixing bowl, blend together the softened cream cheese, sour cream, and Parmesan cheese until smooth and fully combined.

Grease a 9x13-inch baking dish lightly with butter. Place half of the cooked spaghetti in an even layer at the base of the dish.

Spread the cream cheese mixture evenly across the pasta layer, ensuring full coverage.

Add the remaining spaghetti on top of the cream layer, followed by the warm meat sauce, spreading it out to cover all the noodles.

Sprinkle the shredded mozzarella generously over the surface. Distribute the small butter cubes across the top to enhance richness during baking.

Cover the dish loosely with foil and bake in a preheated oven at 350°F (175°C) for 20 minutes.

Uncover and continue baking for an additional 10 minutes, or until the top is golden and the cheese has melted and begun to bubble.

Let the casserole sit for 5 minutes before cutting into portions. Optionally, garnish with a scattering of freshly chopped parsley before serving.