Melt the butter in a heavy-bottomed saucepan or small Dutch oven over medium heat. Add the finely chopped celery and green onions. Saute for 3 to 4 minutes until they become tender and translucent. Do not let the vegetables brown.

Sprinkle the all-purpose flour directly over the softened vegetables. Stir constantly for exactly 60 seconds to completely cook out the raw flour flavor and create a smooth, bubbling blond roux paste.

Pour the whole milk into a microwave-safe container and heat for 60 to 90 seconds until warm to the touch. Pro Tip: This is a crucial temperature-matching step. Pouring ice-cold milk directly into a hot butter-flour roux will shock the fats, creating stubborn starch lumps. Introducing warm milk allows the roux to dissolve smoothly into a velvety liquid instantly.

Whisk the warm milk into the saucepan in a steady stream until completely uniform. Add the heavy whipping cream, Old Bay seasoning, and the 1 tablespoon of tomato paste.

Whisk aggressively for 1 minute until the dense tomato paste is fully dissolved, turning the milk matrix a uniform, classic light pink-orange color. Bring the liquid to a very low simmer—never allow it to reach a rapid boil, which can snap the dairy emulsion and cause the sauce to separate.

Cook over low heat for 5 to 7 minutes, stirring frequently with a flat-edged wooden spoon to prevent the starches from sticking and scorching on the bottom of the pot, until the soup thickens enough to coat the back of your spoon.

The Residual Protein Steep: Turn the stove burner completely off. Gently fold the bite-sized shrimp pieces and the drained lump crab meat directly into the hot cream base. Immediately slide the pot off the hot burner, pop the lid on tight, and let it sit undisturbed for 3 minutes. Crucial: Seafood cooks incredibly fast. Allowing the crab and shrimp to heat through purely via the residual energy trapped in the thick bisque prevents them from seizing, ensuring the crab remains flakey and the shrimp stays tender instead of turning rubbery. Garnish with fresh parsley and serve hot.