Place leek in an 8- to 10-quart pan. Add onions, carrots, celery, mushrooms, tomatoes, fennel, parsley, thyme, oregano, peppercorns, salt and enough water to cover vegetables (about 2 qt.). Bring to a boil over high heat, then reduce heat and simmer, uncovered, for 2 hours, adding water as needed to keep vegetables barely covered.
Line a colander with a layer of cheesecloth (or use a large, fine wire strainer); set over a large bowl. Pour stock mixture into colander and drain vegetables well; discard vegetables. Reserve stock for use.
For Sweet Potato, Chipotle and Apple Soup:
In a medium soup pot, heat 2 tablespoons of the oil until shimmering. Add the onion, garlic and ginger and cook over low heat, stirring, until softened, about 7 minutes. Add the apples and celery and cook for 5 minutes. Add the sweet potatoes and cook for 5 minutes. Add the stock and bring to a boil. Cover partially and simmer over low heat until the fruit and vegetables are very tender, about 45 minutes. Stir in the chipotle.
Working in batches, puree the soup in a blender until smooth. Season with salt and white pepper and return to the pot.
Serve the soup in shallow bowls (or a coffee cup as I did in the photo) and garnish with the fried tortilla strips and cilantro, if desired.