Preheat the oven to 375°F. Prepare a large pot of boiling water and cook the spaghetti according to the package instructions. While the water heats, proceed with the steps below.
In a large nonstick skillet over medium heat, brown the ground beef until fully cooked. Drain any excess grease.
Stir in the diced onion, bell pepper, and seasonings. Sauté until softened, about 5 minutes. Add the minced garlic and cook for an additional minute.
Lower the heat and mix in the marinara sauce, ricotta cheese, Parmesan, and cream cheese. Let the mixture warm through.
Combine the cooked spaghetti with the sauce mixture using kitchen tongs, ensuring the pasta is evenly coated. Note that the dish will appear saucy, but the pasta will absorb more as it bakes.
Grease a 9 x 13-inch baking dish lightly. Layer half of the spaghetti mixture into the dish and sprinkle 2 cups of shredded mozzarella cheese on top. Add the remaining spaghetti, followed by the rest of the mozzarella.
For immediate serving, bake uncovered for 15 minutes, or until the cheese is melted and bubbly. Allow it to rest for 5 minutes before serving. Pair with garlic bread for a complete meal.