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 wild rice and mushroom soup:
If you are not using pre-cooked wild rice: put wild rice, salt, and 8 cups cold water in a medium pot. Bring to a boil, lower heat to maintain a steady simmer, and cook until rice is tender, about 45 minutes. Drain and set aside.
Meanwhile, put dried porcini in a small bowl and pour in 1 1/2 cups boiling water. Let sit until soft, about 15 minutes.
In a large pot, cook 1 tbsp butter (and pancetta, if using) over medium-high heat until the meat renders some of its fat and turns a lighter pink.
Add button mushrooms and leek, Cook, stirring occasionally, until mushrooms give off their liquid, about 10 minutes.
Meanwhile, lift porcini from liquid with a slotted spoon (reserving liquid), chop finely, and add to pot.
Sprinkle sauteed vegetable mixture with flour and cook, stirring constantly, until flour starts to stick to the bottom of the pot (scrape it up as much as possible while stirring). Add wine, reserved liquid from soaking porcini (pouring slowly so as to leave any grit behind), and stock. Bring to a boil, then lower heat to maintain a steady simmer and cook 15 minutes.
Meanwhile, combine remaining 4 tbsp butter, the parsley, and pepper, Set aside.
Add reserved wild rice to vegetable mixture and cook 10 minutes. Stir in cream and cook until hot, about 1 minute. Divide soup among 8 bowls and serve hot, with a dollop of parsley butter on each serving.