Step 1: Prepare the Pasta
Bring a large pot of salted water to a rolling boil. Add spaghetti and cook according to package directions until al dente (usually 8-10 minutes). Drain in a colander and toss with olive oil to prevent sticking. Set aside.

Step 2: Make the Meat Sauce
While pasta cooks, heat a large skillet over medium-high heat. Add ground beef and cook for 5-7 minutes, breaking it apart with a wooden spoon until browned and no longer pink. Drain excess fat, leaving about 1 tablespoon in the pan. Add diced onion and cook for 3-4 minutes until softened. Stir in minced garlic and cook for 1 minute until fragrant.

Step 3: Build the Sauce Flavor
Add marinara sauce, diced tomatoes with their juices, tomato paste, Italian seasoning, basil, red pepper flakes (if using), salt, pepper, and sugar. Stir well to combine. Reduce heat to low and simmer for 10-12 minutes, stirring occasionally, allowing flavors to meld together. Taste and adjust seasonings as needed.

Step 4: Create the Creamy Cheese Mixture
In a large mixing bowl, combine softened cream cheese, sour cream, 1 cup of mozzarella, cheddar cheese, ½ cup Parmesan, eggs, and chopped parsley. Mix until smooth and well combined. This creates the signature creamy layer.

Step 5: Assemble the Casserole
Preheat your oven to 350°F (175°C). Lightly grease your 9×13-inch baking dish with cooking spray or butter. Spread half of the cooked spaghetti evenly across the bottom of the dish. Pour half of the meat sauce over the pasta, spreading it to cover. Dollop and spread the entire cream cheese mixture over the meat sauce layer. Top with remaining spaghetti, then pour remaining meat sauce over everything.

Step 6: Add the Cheese Topping
Sprinkle the remaining 1 cup mozzarella and ¼ cup Parmesan evenly over the top of the casserole. This creates that gorgeous golden, bubbly cheese layer everyone loves.

Step 7: Bake to Perfection
Cover the baking dish with aluminum foil (spray the underside with cooking spray to prevent cheese from sticking). Bake for 30 minutes covered. Remove foil and bake for an additional 15-20 minutes uncovered, until the cheese is melted, bubbly, and golden brown around the edges.

Step 8: Rest and Serve
Remove from oven and let the casserole rest for 10 minutes before serving. This allows the layers to set and makes cutting and serving much easier. Garnish with fresh parsley and extra Parmesan if desired. Serve hot and enjoy!