Frozen mixed berries make it easy to throw together this simple fruit dessert. A mixture of oats, spices and sugar does double duty here. We add a small amount to the berries to sweeten and thicken the filling; into the remainder we add almonds and softened butter to make a crumble topping that bakes up toasty and crisp, a delicious flavor and textural contrast to the soft, juicy berries. Quick-cooking oats are best here; old-fashioned oats remain too firm and chewy. Serve warm or at room temperature, ideally with vanilla ice cream.