Put the peppercorns, coriander seeds, cumin seeds, and fennel seeds in a small cast-iron frying pan and set over medium-high heat. Stir and roast until the spices emit a roasted aroma and some turn a shade darker. Empty into a plate to cool, then grind in a clean coffee grinder or other spice grinder. (It is a good idea to sift the ground spices through a fine sieve, stirring them about with a spoon as they pass slowly through the mesh. This is not absolutely necessary, but it makes for a finer soup.) Add the turmeric and cayenne to the spice mixture.
Put the chickpea flour in a bowl. Slowly add 2 tablespoons of the stock, mixing as you go. Add another 4 cups of stock and mix.
Combine the chickpea flour mixture, spices, all the vegetables, the curry leaves, garlic, onion, and ginger in a large pan and bring to a boil. Cover, turn the heat down to low, and simmer for about 50 minutes, or until all the vegetables are tender. Blend the soup in a blender in several batches, if necessary, and then press through a coarse sieve. Return the soup to the soup pan, add the coconut milk and salt, and bring to a simmer. Simmer gently for 2 to 3 minutes to blend the flavors; thin out with more stock, as needed. Serve hot with lime wedges.
Recipe courtesy of Madhur Jaffrey, World Vegetarian, Clarkson Potter, 1999