Although this soup is full of earthy flavor, it's also relatively light, which makes it a perfect first course for the big holiday meal. As a bonus, it can be prepared several days ahead and quickly reheated at the last minute.
03In same stock pot over moderate heat, heat remaining 2 tablespoons butter. Stir in leeks, cover, and cook, stirring often, until soft, about 5 minutes. Add cooked mushrooms, sprinkle with flour, and stir until flour is evenly distributed. Stir in stock, then salt, pepper, and remaining 1 teaspoon thyme. Bring to boil, stirring often, then reduce heat to low, set lid ajar, and simmer 20 minutes. (Soup can be made up to 2 days ahead and refrigerated, covered. Reheat before serving.)
04Divide soup among 8 bowls and top each portion with dollop of thyme cream. Serve immediately.