How to Make It
Melt the butter in a large saucepan. Add the chicken and cook over moderately high heat, stirring, until golden, about 2 minutes. Transfer to a plate. Add the onion and all but 1 tablespoon of the leeks and cook, stirring often, until softened, about 5 minutes. Add the sweet potatoes and cook for 3 minutes. Return the chicken to the pan. Add the stock, 1 tablespoon of the sage, the parsley, bay leaf, salt and white pepper and bring to a boil. Cover and simmer over moderately low heat, until the chicken is white throughout and the potatoes are tender, about 10 minutes.
Using a slotted spoon, transfer 1 1/2 cups of the sweet potatoes to a food processor; puree until smooth. Return the puree to the saucepan. Stir in the milk and season with salt and pepper. Simmer the chowder for 3 minutes, then discard the bay leaf. Serve the chowder hot, garnished with the remaining 1 tablespoon each of leeks and sage.