Cream of Cope's Corn Soup with Shrimp and Wild Mushrooms

Cook onion in butter in a 5- to 6-quart heavy pot over medium-low heat, stirring, until softened. Add corn, heavy cream, and 6 cups stock and bring to a boil.
Wrap parsley, thyme, bay leaves, and peppercorns in cheesecloth and tie with string to make a bouquet garni. Add to pot with 1 teaspoon salt and 1/2 teaspoon pepper and simmer, covered, 45 minutes.
Discard bouquet garni, then purée corn mixture in batches in a blender until smooth (use caution when blending hot liquids), about 2 minutes per batch. Strain soup as puréed through a fine-mesh sieve into a large heavy saucepan, pressing on and then discarding solids.
Return soup to a simmer, stirring occasionally, and keep warm over low heat. Thin with additional stock if necessary.
Trim mushrooms, discarding any dark gills and tough stems, then slice or tear into bite-size pieces.
Heat 2 tablespoons butter in a 10-inch heavy skillet over medium heat until foam subsides, then cook mushrooms, stirring, until tender and golden, about 8 minutes. Season with salt and pepper.
Transfer mushrooms to a bowl and melt remaining 1 1/2 tablespoons butter in skillet. Meanwhile, toss shrimp with 1/4 teaspoon each of salt and pepper. Cook shrimp in butter, turning occasionally, until barely cooked through, 3 to 4 minutes. Return mushrooms to skillet and cook, stirring, 1 minute. Remove from heat and stir in chives and salt and pepper to taste.
Mound shrimp and mushrooms in shallow bowls, then add soup and top each serving with a dollop of crème fraîche.