Place flour in a 9-inch cast-iron skillet; cook over medium heat 15 minutes or until brown, stirring constantly with a whisk. Remove from heat.
Melt the butter in a Dutch oven coated with cooking spray over medium-high heat. Add carrot and the next 5 ingredients (carrot through garlic), and sauté 5 minutes or until vegetables are tender. Add Old Bay seasoning, salt, black pepper, dried thyme, and bay leaf; cook for 1 minute. Sprinkle the browned flour over vegetable mixture, and cook 1 minute, stirring frequently. Stir in clam juice, and bring the mixture to a boil. Reduce heat, and simmer 10 minutes or until mixture is slightly thick, stirring frequently.
Stir in the milk and half-and-half; cook 4 minutes. Stir in crabmeat and sherry; cook 5 minutes or until the soup is thoroughly heated. Discard bay leaf before serving.