1. Preheat oven to 400 degrees F. Coat a baking sheet with cooking spray.
2. Arrange the polenta on the prepared baking sheet and bake until crispy on the bottom and heated through, about 20 minutes.
3. Meanwhile, heat oil in a large skillet over medium-high heat. Add onion and cook, stirring often, until lightly browned, 2 to 3 minutes. Stir in mushrooms, salt and pepper, and cook, stirring often, until the mushrooms are softened and most of the liquid has evaporated, 8 to 10 minutes.
4. Pour in wine; bring to a boil and scrape up any browned bits from the bottom of the pan. Reduce the liquid until the pan is almost dry, 2 to 3 minutes. Stir in sour cream and bring to a simmer. Remove from the heat, stir in cheese and tarragon. Serve the sauce over the polenta.

Yield: 4 servings