Trim the leaves off 1 large or 2 medium leeks. Cut the leeks in half lengthwise and rinse under water to remove silt. Chop finely.
Cut the green stems off 1 fennel bulb. Separate the sheaths of the bulb and remove the core. Finely chop or dice the fennel.
Optional: Boil the fennel stems and any fronds in 8 C. of water with a small bunch of fresh herbs to make a stock.
Dice 3-4 carrots to make 2 C.
Saute the leeks, fennel bulb and carrots in 3 T. olive oil with a dash of hot pepper flakes and 1/2 t. dried herbs (thyme, rosemary, and/or marjoram) until very soft and beginning to brown.
Mince 3 cloves of garlic. Wash, trim and chop 2 C. fresh oyster mushrooms (or other soft mushroom). Add to the pot and saute for 3 minutes.
Strain the fennel from the broth and then pour 6 C. of the liquid into the vegetables. (Or use 3 C. water and 3 C. vegetable broth). Bring to a boil.
Wash and dice potatoes to make 2 C. and add to the pot.
Dice 1 14 oz. can of tomatoes and add them to the pot along with their juices. Season generously with salt and pepper. Add additional broth if the soup is too thick.
Lower the heat to a simmer and cook until the potatoes are tender.