Preheat the oven to 350°F (175°C). Lightly butter a 9x9-inch baking dish or coat it with nonstick spray.

If using raw shrimp, poach them in boiling water until pink, about 2 to 3 minutes. Drain, then chop into bite-size pieces. Pat the crab meat dry with paper towels and remove any shell bits.

Melt the butter in a medium saucepan over medium heat. Whisk in the flour and cook for about 2 minutes, stirring constantly, until the mixture turns pale golden and smells nutty.

Slowly pour in the room-temperature milk while whisking vigorously to keep the sauce smooth. Continue whisking until the sauce thickens and coats the back of a spoon, about 5 to 7 minutes. Add a splash more milk if it becomes too thick.

Remove the saucepan from the heat. Stir in the cheddar, Parmesan, and softened cream cheese until fully melted and smooth. Season with garlic powder, onion powder, seafood seasoning, salt, black pepper, and cayenne if using. Taste and adjust the seasoning.

Gently fold the crab meat and chopped shrimp into the sauce. Add the Meyer lemon juice and chopped parsley, stirring just until combined so the crab stays in nice chunks.

Pour the seafood mixture into the prepared baking dish and smooth the top into an even layer.

Sprinkle the panko breadcrumbs evenly over the casserole. For extra richness, toss the crumbs with a little melted butter or olive oil before adding them on top.

Bake for 25 to 30 minutes, until the topping is golden brown and the sauce is bubbly around the edges.

Let the casserole rest for 5 minutes before serving. Garnish with additional chopped parsley if desired.