In a medium saucepan, melt the butter over medium heat.
Add the chopped celery and green onions and sauté until softened, about 3–4 minutes.
Stir in the flour and cook for 1–2 minutes, whisking constantly to remove the raw flour taste.
Warm the milk in the microwave or on the stove, then slowly pour it into the saucepan, whisking continuously to prevent lumps.
Whisk in the heavy cream, tomato paste, Old Bay seasoning, and salt and pepper to taste.
Bring the mixture to a low simmer over medium-low heat, stirring frequently, until the bisque thickens, about 5–7 minutes.
Gently fold in the shrimp and crab meat, cooking until heated through, about 3–5 minutes.
Serve the bisque hot, garnished with fresh parsley.
Enjoy!