I let it sit in the fridge for a night so it would thicken a bit and then rewarmed it the next day, adding the thyme cream (it doesn't need to whipped, necessarily). Great soup and easy to make. Getting mushrooms already sliced really cuts down on the prep time.

The recipe is from Epicurious.com

Ingredients
1/2 cup heavy cream
3 teaspoons fresh thyme, chopped (or to taste)
6 tablespoons (3/4 stick) unsalted butter
2 pounds baby bella mushrooms (or crimini, or both), cut into 1/4-inch-thick slices (10 1/2 cups)
3 large leeks, white and pale green parts only, cut into 1/4-inch dice (2 1/2 cups)
6 tablespoons all-purpose flour
6 cups veggie broth
1 1/2 teaspoons salt (or to taste)
3/4 teaspoon freshly ground black pepper

Preparation
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.

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.

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.)

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

Test-Kitchen Tip:
Leeks can be gritty—they need to be washed well before cooking. Chop the white and pale green parts only (the dark green top is bitter and should be discarded), and place them in a bowl of cold water. Stir them briskly to loosen dirt, then let them stand for a few minutes so the grit can sink to the bottom of the bowl. Lift the leeks out of the water, leaving the grit behind, and transfer them to a sieve to drain.