First, preheat your oven to 350°F (175°C). Lightly grease a 9×13 inch baking dish and set it aside.

Bring a large pot of salted water to a boil. Cook the pasta according to the package instructions until al dente. Drain well and set aside to cool slightly.

Meanwhile, in a medium saucepan over medium heat, melt the butter. Once melted, whisk in the flour and cook for about one minute, stirring constantly to create a roux. Gradually add the milk while continuing to whisk until the mixture becomes smooth and thickened.

Remove the saucepan from the heat and stir in the sour cream, mayonnaise, Old Bay seasoning, garlic powder, and black pepper until combined. Taste and adjust seasoning if needed.

In a large mixing bowl, combine the cooked pasta, crab meat, half of the mozzarella cheese, half of the Parmesan cheese, and the chopped green onions. Pour the sauce over the pasta mixture and gently stir until everything is evenly coated.

Transfer the mixture to the prepared baking dish. Sprinkle the remaining mozzarella and Parmesan cheese evenly over the top. If using, sprinkle the breadcrumbs over the cheese for extra crunch.

Bake for 20–25 minutes or until the cheese is melted, bubbly, and golden brown on top. Let the Crab Pasta Bake rest for a few minutes before serving to allow the flavors to settle and make slicing easier.