In a large pot melt Unsalted Butter (3 tablespoon) over medium heat. Add White Onion (1 cup), Celery (1 cup), Fennel Bulb (1 cup) and Garlic (1 clove). Sauté until starting to soften but not brown, about 10 minutes.
While vegetables are cooking prepare Corn Kernels (4 cup) by peeling off corn husks and removing silk. Stand corn on end and remove kernels by running a large knife along the body, cutting at the base of the kernels. Trim ends off bare cobs and cut the cob in half. Add kernels and cobs to pot along with Vegetable Broth (2 cup), Water (2 cup), Fresh Thyme (to taste), and Salt (1/2 teaspoon).
Bring to a simmer, cover and cook for 30 minutes. Remove cobs and thyme sprigs and discard. Allow to cool at room temperature for 10-20 minutes. Add remaining Salt (1/2 teaspoon) and adjust as needed, remembering that cold foods tend to taste saltier than warm.
Using a blender, blend soup in batches until smooth. Pour through a fine mesh sieve, pressing against the sides to extract all the soup. Cover and chill until cool, stirring occasionally.
Gently stir together Lump Crab Meat (1 cup), Fresh Chives (2 tablespoon), Fennel Leaves (2 tablespoon), Fennel Bulb (4 tablespoon), Lemon Juice (3 teaspoon), Dijon Mustard (2 teaspoon), and Salt and Pepper (to taste). Cover and keep chilled until ready to use. Can be made up to several hours ahead.
Scoop approximately 1/4 cup of crab salad into the centre of shallow serving bowls. Ladle soup around crab salad and drizzle with a spoon of Sour Cream (1/3 cup) swirling into soup slightly. Serve immediately.