In a medium-sized saucepan, melt the butter over medium-high heat.

Add the chopped celery and sliced green onions to the pan.

Cook the vegetables, stirring occasionally, until they soften.

Sprinkle in the flour, stirring continuously, and cook for about a minute.

Slowly pour in the warm milk, whisking constantly to avoid lumps.

Add the heavy cream, tomato paste, and Old Bay seasoning, mixing well.

Reduce the heat to low and simmer the mixture until it starts to thicken.

Gently fold in the crab meat and shrimp, cooking just until the seafood is heated through.

Serve the bisque hot, garnished with a sprinkle of fresh parsley if desired.