01 - 
                Preheat your oven to 375°F (190°C). Grease a 9x13-inch baking dish and set aside.
              
              
              
                02 - 
                In a large skillet, melt the butter over medium heat. Add the chopped onion, bell pepper, celery, and garlic. Cook until the vegetables are softened, about 5-7 minutes.
              
              
              
                03 - 
                Stir in the flour and cook for another 2 minutes, stirring constantly.
              
              
              
                04 - 
                Gradually whisk in the milk and chicken broth until smooth. Cook, stirring constantly, until the mixture thickens, about 5 minutes. Season the sauce with Old Bay seasoning, salt, and pepper to taste.
              
              
              
                05 - 
                Gently fold in the lump crabmeat and shrimp, making sure they are evenly coated with the sauce.
              
              
              
                06 - 
                Transfer the mixture to the prepared baking dish and spread it out evenly. Sprinkle the shredded cheddar cheese over the top of the casserole, followed by the breadcrumbs.
              
              
              
                07 - 
                Bake in the preheated oven for 25-30 minutes, or until the casserole is bubbly and golden brown on top.
              
              
              
                08 - 
                Remove from the oven and let it cool for a few minutes before garnishing with chopped fresh parsley and serving.