Prepare ingredients first: peel and devein the shrimp, pick crab meat clean of shells, and finely chop the onion, celery, and carrot. Mince the garlic and measure flour and stock so everything is at hand.
In a large pot, melt the butter over medium heat until foaming but not browned.
Add the chopped onion, minced garlic, celery, and carrot. Sauté, stirring occasionally, until softened and fragrant, about 5 minutes. Do not let the garlic burn.
Sprinkle in the flour and stir constantly for 1 to 2 minutes to cook the raw flour taste and form a light roux. The vegetables will be lightly coated.
Gradually pour in the warm seafood stock while whisking to avoid lumps. Continue whisking until the mixture is smooth.
Bring the mixture to a gentle simmer, stirring occasionally. Allow it to thicken slightly for about 5 minutes.
Add the shrimp and crab meat to the simmering soup. Cook for about 3 to 5 minutes until the shrimp turns pink and opaque. Be careful not to overcook the shrimp—small shrimp cook quickly.
Reduce heat to low and stir in the heavy cream. Warm through gently without boiling, which can cause cream to separate.
Season with salt and pepper to taste. Taste and adjust seasoning—a small squeeze of lemon can brighten flavors if desired.
Remove from heat. Ladle into bowls, garnish with chopped fresh parsley, and serve hot with crusty bread or your favorite sides.
Notes
Store leftovers in an airtight container for up to 3 days. Cool to room temperature before refrigerating.