Melt butter in a large pot over medium heat. Add onion and celery and cook until softened.
Stir in garlic and cook until fragrant, about 30 seconds.
Sprinkle in flour and stir continuously for 1–2 minutes to form a light roux.
Slowly whisk in seafood stock until smooth. Bring to a gentle simmer.
Use an immersion blender to partially blend the soup, leaving some texture.
Stir in heavy cream, paprika, white pepper, cayenne, and salt. Reduce heat to low.
Add shrimp and crab meat. Simmer gently until shrimp are pink and cooked through.
Remove from heat and let rest 5 minutes before serving. Garnish and enjoy.