Make the recipe in advance in the slow-cooker insert and pop into the slow cooker just before guests start arriving. This recipe works best on high for shorter periods of time. Long cooking times (over 4 hours) lead to the fruit losing some of its brightness of color and flavor. You may cook it on low for 3 to 4 hours if needed, adding a layer of peeled, sliced Granny Smith apples first, as they are sturdier and will protect the softer peaches and berries from the heat. Surprisingly, both frozen and fresh fruit work almost equally well (frozen was a tiny bit inferior in terms of texture and extra liquid it threw off).