Cover dried mushrooms with 2 cups boiling water in a heatproof bowl; let stand until soft, about 30 minutes. Using a slotted spoon, transfer mushrooms to a cutting board, and finely chop; set aside. Strain soaking liquid; set aside.
Melt butter in a large saucepan over medium heat. Add leeks; cook, stirring occasionally, until soft but not browned, 10 to 15 minutes. Stir in flour; cook 2 minutes. Add sherry; whisk until smooth. Whisk in stock, thyme, and reserved soaking liquid; bring to a boil. Reduce heat; simmer 5 minutes. Stir in dried and fresh mushrooms. Remove from heat.
Pour into a large bowl. Working in 3 batches, puree soup in a food processor, making sure there are no remaining mushroom pieces; return to pan. Bring to a simmer; cook, stirring occasionally, 15 minutes. Season with salt and pepper.
Stir in cream; bring just to a simmer. Season with salt. Stir in milk; cook until just heated through. Serve immediately, or refrigerate, covered, up to 3 days; reheat before serving. To serve, divide among bowls; spoon about 2 teaspoons chervil cream into each center, and sprinkle with croutons.