Melt the butter in a large saucepan. Add the chicken and cook over medium-high heat, stirring, until golden, about 2-3 minutes. Transfer to a plate.
Add the onions 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 and milk to a food processor; puree until smooth. Return the puree to the saucepan.
Season with salt and pepper. Simmer the chowder for 3 minutes, then discard the bay leaf.
Serve the chowder hot, garnished with the remaining leeks and sage.
Cook's Note: Turkey breast can easily be used in place of chicken.