In a large, heavy pot over medium heat, heat butter. Add onion and celery and cook until soft, about 5 minutes. Season with salt, pepper and Old Bay, then stir in garlic and tomato paste. Cook until garlic is fragrant and tomato paste coats vegetables, about 2 minutes. Sprinkle over flour and cook until absorbed, 1 minute more.
Pour in vegetable broth and wine, then stir in bay leaf. Reduce heat and let simmer until liquid is reduced and flavors meld, stirring occasionally, 30 minutes.
Remove bay leaf and puree soup with an immersion blender on high (if you do not have an immersion blender, puree in batches in a regular blender) until very smooth. At this point, you can chose to thicken the bisque more with the cornstarch and water. Make a mixture of the two and bring soup to a boil. Stir in cornstarch slurry and reduce hit and let simmer and thicken for about a minute. Return to medium low heat and stir in half and half and the crab, reserving 4 Tablespoons of the crab for garnish. Cook until just warmed through, about 5 minutes.
Divide among bowls and garnish with remaining crab and parsley.
