Heat olive oil in a large skillet over medium-high heat. Add the ground beef and diced onion. Cook for 7–9 minutes, breaking the beef into small crumbles, until browned and fully cooked. Drain excess grease.

Stir in minced garlic, Italian seasoning, dried basil, salt, black pepper, and optional red pepper flakes. Cook for another 30–60 seconds until fragrant.

Add marinara sauce, crushed tomatoes, and tomato paste to the skillet. Stir thoroughly and simmer for 5–7 minutes until the sauce thickens slightly. Remove from heat.

Bring a large pot of salted water to a boil. Add the spaghetti and cook for approximately 2 minutes less than the package's al dente instructions. Drain well and toss with the melted butter to prevent sticking.

In a medium mixing bowl, combine softened cream cheese, sour cream, cottage cheese, Parmesan cheese, garlic powder, and Italian seasoning. Mix until smooth and creamy.

Lightly grease the inside of a 6-quart oval slow cooker with nonstick cooking spray or butter.

Spread approximately one-quarter of the prepared meat sauce across the bottom of the slow cooker.

Arrange half of the cooked spaghetti over the meat sauce, spreading the noodles evenly without pressing them down too tightly.

Spoon the cream cheese mixture over the spaghetti and gently spread it into an even layer. Sprinkle ½ cup shredded mozzarella cheese over the filling.

Spoon another one-quarter of the meat sauce over the cheese mixture. Add the remaining spaghetti and spread evenly.

Pour the remaining meat sauce over the spaghetti, making sure the noodles are well covered to prevent drying out.

Sprinkle the remaining 1½ cups mozzarella cheese and 1 cup shredded sharp cheddar cheese evenly over the top.

Cover and cook on LOW for approximately 2–3 hours, until the casserole is heated throughout, the sauce is gently bubbling around the edges, and the cheese has completely melted. Begin checking after 1½ hours to avoid overcooking the pasta.

Turn off the slow cooker and let the spaghetti rest uncovered for 10–15 minutes. This helps the creamy layers settle and makes serving easier.

Sprinkle freshly chopped parsley over the melted cheese. Use a large serving spoon or pasta server to lift generous portions, making sure each serving includes spaghetti, savory meat sauce, and creamy cheese filling.