2 tbsp 30 ml) butter, 3 cups (750 ml) chopped leeks (white and light green parts), 1/2 tsp (2 ml) dried thyme or marjoram, 4 cups (1 l) sliced mixed mushrooms (button, cremini, shiitake, oyster), 3 cups (750 ml) chicken or vegetable stock, 1 cup (250 ml) milk, 1 tbsp (15 ml) white or red wine vinegar, Salt, Pepper, 1 cup (250 ml) 35 % whipping cream, 1 tbsp (15 ml) finely chopped fresh chives.
In a pot, melt butter over medium heat; cook leeks and thyme, stirring, for about 5 min or until soft. Increase heat to medium-high; add mushrooms and cook, stirring, for about 8 min or until golden. Add stock; bring to a boil. Reduce heat and simmer for 5 min or until vegetables are soft. In blender or food processor, in batches, or with immersion blender, purée until smooth. Return to pot (if necessary).
Stir in milk and reheat over medium-low heat until steaming. Do not boil. Stir in vinegar; season to taste with salt and pepper. Just before serving, in a bowl, whip the cream until thickened but not stiff. Stir in chives. Gently stir into soup.