Step 1: Preheat your oven to 350°F (175°C). Lightly grease a 9x13 inch baking dish.
Step 2: Cook the spaghetti according to package directions until al dente. Drain well and set aside.
Step 3: While the spaghetti cooks, prepare the meat sauce. In a large skillet or Dutch oven, brown the ground beef (and sausage, if using) over medium-high heat. Drain any excess fat.
Step 4: Add the chopped onion and minced garlic to the skillet with the browned meat. Cook until the onion is softened and translucent, about 5-7 minutes.
Step 5: Stir in the crushed tomatoes, tomato paste, Italian seasoning, dried oregano, red pepper flakes (if desired), salt, and black pepper. Bring to a simmer, then reduce heat to low, cover, and let it cook for at least 15-20 minutes to allow the flavors to meld.
Step 6: In a medium bowl, combine the ricotta cheese, softened cream cheese, 1/2 cup grated Parmesan cheese, an egg, and a pinch of salt and pepper. Mix until smooth and well combined.
Step 7: Spread half of the cooked spaghetti evenly in the prepared baking dish.
Step 8: Top the spaghetti layer with all of the creamy ricotta mixture, spreading it gently to cover the pasta.
Step 9: Spoon all of the meat sauce evenly over the cheese layer.
Step 10: Top the meat sauce with the remaining half of the cooked spaghetti.
Step 11: Finish with a generous layer of shredded mozzarella cheese.
Step 12: Cover the baking dish loosely with aluminum foil and bake for 25 minutes.
Step 13: Remove the foil and bake for an additional 15-20 minutes, or until the cheese is bubbly and lightly browned. Let the million dollar spaghetti recipe rest for 10-15 minutes before serving to allow it to set for cleaner slices.