Asparagus & leek bisque

Butter, unsalted	4 tablespoons
Leeks, cleaned, white parts thinly sliced	1 cup
Asparagus, tough ends discarded, cut into 1-inch pieces	3 pounds
Pepper, white, freshly ground	¼ teaspoon
Flour, all-purpose	3 tablespoons
Chicken stock, low sodium	1 quart
Sherry	½ cup
Cream, heavy	2 cups
Crab, lump (optional)	1 pound
Salt

1. In a heavy saucepan, melt the butter over medium heat. 
2. Add the leeks, and sauté until softened, about 4 minutes. 
3. Add the asparagus and pepper; sauté 4 minutes. 
4. Add flour; sauté until the mixture begins to bubble and starts to brown slightly. 
5. Gradually stir in stock, and then add the sherry. 
6. Simmer until the asparagus is tender, about 20 to 25 minutes. 
7. Remove the pan from the heat, and purée with a stand blender or hand-held immersion blender. 
8. Return the pan to heat, add the cream and crab, and stir until heated; do not boil. 
9. Season with salt, and serve immediately.