In a large, heavy pot over medium heat, melt 3 tablespoons of the butter. When hot, add the leeks and sauté, stirring until softened, 4 to 5 minutes. Add the asparagus and stir and cook 1 minute more.

Add broth and bring mixture to a simmer. Reduce the heat to low and cook at a simmer until vegetables are very tender, 15 to 20 minutes. Remove from heat. Purée the mixture in batches in a food processor, blender or food mill, then return to the pot. (Or use an immersion blender to purée the soup in the pot.)

Whisk in crème fraiche, lemon juice and cayenne. Season the bisque with salt to taste. (Soup can be prepared 2 days ahead. Cook to this stage; cool, cover and refrigerate. Reheat, stirring, over medium heat.) Return pot to very low heat to keep warm while finishing the soup.

In a medium, heavy skillet over medium-high heat, heat remaining tablespoon of butter and the canola oil. When hot, add scallops, and cook until light golden brown and just cooked through, about 1 to 1½ minutes per side, depending on their size. Season with salt and pepper.

Ladle soup into shallow soup bowls. Place a few scallops atop each serving, and garnish with a sprinkle of breadcrumbs and chives.