Preparation:

• Cut the mushrooms arbitrarily.
  
• In 3 l. pan, over medium heat, heat vegetable oil with butter. When the butter stops foaming, add onions and leeks. Add salt and sugar, fry the onion without covering it, stirring occasionally, for 12-15 minutes, until the onion begins to caramelize (change its color to golden).
• Add the mushrooms to the onion and simmer for another 5 minutes until the mushrooms release juice. Pour in the broth and milk, bring to a boil, reduce the heat, add salt and pepper, thyme (optional). Cook on low heat for 10 minutes without covering.
• Turn off the soup, remove the sprigs of thyme, if used, and beat with a blender until smooth. Add cream, bringing the soup to the desired consistency. Taste the soup. If necessary, season with salt and pepper.
• Strain the soup through a sieve into a clean saucepan. You can skip this step, but rubbing gives the soup a velvety texture.