Add the butter to the Dutch oven and melt over medium-high heat. Add the leeks, onion, and 1/4 teaspoon salt and cook until softened, about 7 minutes. Stir in the flour. Whisk in the sherry, scraping up any browned bits. Stir in the broth, milk, thyme, and bay leaves. Nestle the chicken, with any accumulated juices, into the pot. Cover and simmer until the chicken is fully cooked and tender, about 1 hour.