Melt 2 tablespoons butter in a pot over medium-high heat and cook until lightly browned. Add the ham and cook until slightly browned, about 3 minutes. Remove the ham with a slotted spoon to a plate. Add 1/2 tablespoon butter, 1 cup leeks, and salt and pepper to taste. Cook 4 minutes, then add to the plate; set aside.

Add the remaining 1 1/2 tablespoons butter, leeks, 1/2 teaspoon salt, and pepper to taste to the pot; cook until wilted, about 3 minutes. Add the mushrooms and cook 3 minutes. Sprinkle in the flour and cook, stirring, 2 minutes. Stir in 2 tablespoons sherry, if using, scraping up any browned bits, and cook until evaporated. Add the broth and 2 cups water. Cover and bring to a boil, then reduce the heat to low and simmer, uncovered, until the mushrooms are tender, 8 to 10 minutes.

Puree the soup in batches in a blender until smooth, cracking the lid to let steam escape. Return to the pot, whisk in the half-and-half and bring just to a boil. Stir in three-quarters of the ham-leek mixture, the remaining 1 tablespoon sherry, if using, and salt and pepper to taste. Serve topped with the remaining ham and leeks.

Per serving: Calories 368; Fat 21 g (Saturated 12 g); Cholesterol 90 mg; Sodium 1,072 mg; Carbohydrate 23 g; Fiber 3 g; Protein 19 g