Mixed-Berry Slump

INGREDIENTS

• 7 1/2 ounces blueberries (1 1/2 cups)
• 6 ounces raspberries or blackberries (1 1/2 cups)
• 12 strawberries, hulled, and halved or quartered if large (1 1/2 cups)
• 1/2 cup plus 2 tablespoons packed light-brown sugar
• 1/4 cup plus 1 tablespoon water
• 1/2 vanilla bean, split and scraped, pod reserved
• Salt
• 1 tablespoon cornstarch
• 1/2 cup all-purpose flour
• 1/2 teaspoon baking soda
• 1/8 teaspoon freshly grated nutmeg
• 2 tablespoons cold unsalted butter, cut into small pieces
• 1/4 cup buttermilk