Mushrooms are available year-round, but for many, they're one of the flavors most associated with fall, when wild mushrooms hit the markets. There are over 300 types of cultivated mushrooms, not including the numerous varieties of edible wild fungi. Some are mild, some almost tasteless, but others are robust with rich, woodsy flavor.For the fullest flavor, choose large, dark mushrooms with open gills. If you don't want to use heavy cream in your soup, you can thicken the base with a beurre manie - a paste made with equal parts of butter and flour.
Heat the butter in a large skillet, and sauté the shallots and onions over medium-high heat for 4 minutes. Add the garlic, thyme, and mushrooms, and cook, stirring frequently, until the mushrooms are crisp-tender, about 5 minutes.
Add the wine and reduce the liquid by half.
Add the stock and simmer the soup over low heat for 30 minutes.
Add the cream and heat through. Season to taste with salt and pepper. Garnish each serving with a sprinkling of chives, if desired.