Soak dried porcini mushrooms in hot water; cool and refrigerate for up to a week. Drain, and save stock and porcini.
Bring to boil all ingredients, and cook for about 30 min. Drain and discard solids.
Clean mushrooms by immersing them quickly in water and pat dry. Remove stems and gills if using portobello and save them for the stock. Cube mushrooms and set aside. In a large skillet, or Dutch oven, sweat the minced leeks with olive oil and butter for about 10 minutes. Stir in chopped mushrooms (fresh mushrooms and rehydrated porcini), cover and cook for 15 minutes on medium heat. Deglaze with marsala and reduce to dry. Add mushroom stock and milk, and season with salt and pepper to taste. Cover and cook for 30 minutes. Blend half of the soup with 5 Tbsp/75g butter, or heavy cream, or olive oil. Add remaining soup.
Sauté mushrooms and olive oil in a hot frying pan for a few minutes. Add minced shallots and sauté for 5 minutes more. Throw in butter and continue sautéing until golden brown. Season with salt, Panko and parsley. Transfer to a clean plate to stop cooking. Top soup with some sautéed mushrooms. Season with fleur de sel and ground pepper. Add a drizzle of truffle oil and lemon zest if desired. Bon appétit!