Country Mushroom Soup

• 4 servers
• 2 bay leaves
• 425 gr mushrooms (sliced fresh)
• 475 ml chicken broth
• 70 gr celery (finely chopped)
• 55 gr butter
• 45 ml cooking oil
• 32 gr all purpose flour
• 30 gr onion (finely chopped)
• 1 gr salt