Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking dish with cooking spray or butter and set aside.

Heat olive oil in a large skillet over medium-high heat. Add the diced onion and cook for 3-4 minutes until softened. Add the minced garlic and cook for another 30 seconds until fragrant.

Add the ground beef and cook, breaking it up with a wooden spoon, until no longer pink — about 7-8 minutes. Drain excess fat if needed.

Stir in the marinara sauce, Italian seasoning, red pepper flakes (if using), salt, and black pepper. Reduce heat to low and simmer for 10 minutes, stirring occasionally. Remove from heat and set aside.

Cook spaghetti in a large pot of generously salted boiling water for 2 minutes less than the package directions (al dente). Drain well and immediately toss with butter to prevent sticking. Set aside.

In a large mixing bowl, beat the softened cream cheese until smooth. Add the ricotta, egg, garlic powder, and salt. Stir until well combined and creamy. Set aside.

Spread half of the meat sauce evenly across the bottom of the prepared baking dish.

Layer all of the cooked buttered spaghetti over the meat sauce, spreading it into an even layer.

Dollop the cream cheese and ricotta mixture over the pasta and spread it as evenly as possible across the surface.

Sprinkle 1 cup of the shredded mozzarella over the creamy layer.

Spoon the remaining meat sauce evenly over the mozzarella layer.

Top with the remaining 1 cup of mozzarella and all of the grated Parmesan cheese.

Cover the baking dish tightly with aluminum foil and bake at 350°F for 30 minutes.

Remove the foil and bake uncovered for an additional 10-15 minutes, until the cheese is melted, golden, and beginning to bubble around the edges.

For extra browning, broil on high for 2-3 minutes — watch carefully to avoid burning.

Remove from the oven and let the casserole rest for 10 minutes before slicing and serving. Garnish with fresh chopped parsley.