Heat a large soup pan or dutch oven over medium heat. Add butter and olive oil, when melted add onion and sautee until translucent. Add leeks, thyme and garlic, sautee for an additional 10 minutes. Add mushrooms and sautee until cooked, stirring occasionally - about 10 minutes.
Add sherry, allow to cook down for 3 minutes and then add chicken broth and corn starch/water mixture. Bring to a simmer. Remove from heat and add cream. Season with S&P to taste.
Drizzle with truffle oil to serve.
Recipe by Good Thyme Kitchen at https://www.goodthymekitchen.com/wild-mushroom-soup/