Step 1

Using electric mixer, beat cream just until soft peaks form. Fold in 2 teaspoons thyme. Cover and refrigerate at least 2 hours to blend flavors. (Thyme cream can be made up to 1 day ahead. If cream separates, whisk until soft peaks re-form. ) Remove from refrigerator 1 hour before serving to come to room temperature.

Step 2

In 4-quart stock pot over moderately high heat, heat 2 tablespoons butter until hot but not smoking. Working in 2 batches (add 2 more tablespoons butter before second batch), cook mushrooms, stirring occasionally, until lightly browned, about 8 minutes. Transfer to large bowl.

Step 3

In 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.)

Step 4

Divide soup among 8 bowls and top each portion with dollop of thyme cream. Serve immediately.