A chervil cream subtly brightens this mushroom soup, which is also topped by tiny croutons.
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.
The version of this recipe that Martha made on her show differed from the Martha Stewart Living version because it included only 3 tablespoons of all-purpose flour.