Gently wipe the porcini mushrooms with a paper towel and cut into large pieces.
Quickly rinse the button or oyster mushrooms, slice thinly. Split the leek whites lengthwise, wash, dry, and chop into small pieces. Finely chop the onion and garlic.
Heat olive oil in a saucepan over medium heat. Sauté the garlic, onion, and leek for 2 minutes, covered, without browning. Add porcini and other mushrooms, stir well, and sauté for 3 minutes. Season with salt and pepper, pour in chicken broth, bring to a boil, then simmer covered for 30 minutes.
Transfer to a blender and purée until smooth. Stir in fresh cream and adjust seasoning to taste.
Ladle into soup bowls or small tureens. Serve alongside toasted baguette slices for dipping.
