Step 1In a large, wide, heavy-bottomed saucepan heated over medium-high heat until hot, add the butter and oil. Stir in the onion and cook until softened and just beginning to color, 5 to 6 minutes, stirring constantly. Add the mushrooms and continue to cook until the mushrooms are softened and just beginning to color, 8 to 10 minutes, stirring frequently.
Step 2Stir in the wine, scraping any flavoring from the bottom of the pan. Add the vegetable broth and potatoes, cover loosely and simmer until the potatoes are tender, about 15 minutes. If the liquid reduces too quickly, add a little more broth or water and continue cooking.
Step 3Blend the soup, then bring back to a gentle simmer. Stir in the cream and season with 3/4 teaspoon salt and 1/4 teaspoon pepper, or to taste.