Place a medium saucepan on the stove over medium heat.
Add the butter to the saucepan and allow it to melt completely.
Add the chopped celery and chopped green onions to the melted butter.
Sauté the celery and green onions for several minutes, or until they begin to soften.
Add the all-purpose flour to the saucepan.
Stir the flour into the butter and vegetables until evenly combined.
Cook the flour mixture for approximately 1 minute, stirring frequently to prevent burning.
Warm the whole milk in the microwave until it is heated through but not boiling.
Slowly pour the warm milk into the saucepan while whisking continuously.
Continue whisking until the mixture is smooth and no lumps remain.
Add the heavy whipping cream to the saucepan.
Whisk in the tomato paste until it is fully incorporated into the creamy base.
Add the Old Bay seasoning, salt, and ground black pepper.
Stir until the seasonings are evenly distributed.
Bring the bisque to a low simmer.
Continue cooking, stirring frequently, until the bisque thickens to the desired consistency.
Add the cooked shrimp pieces and crab meat to the saucepan.
Stir gently to combine the seafood with the creamy bisque.
Cook for a few additional minutes, or until the shrimp and crab are heated through.
Remove the saucepan from the heat.
Ladle the bisque into serving bowls.
Garnish with fresh parsley, if desired.
Serve warm.