Lightly oil a large oval 4-quart crockpot insert and reserve.
In a large bowl, mix the oats, flour, brown sugar, cinnamon and salt. Melt the butter, margarine or coconut oil and add to the bowl, stir to mix well. Stir in the ginger. Refrigerate while you cut the fruit.
Slice the nectarines into a large bowl, add the berries, vanilla and arrowroot and toss to mix. Pour into the slow cooker.
Sprinkle the oat mixture over the fruit, close the cooker.
Cook on low for 3 hours. Uncover and take the crock out of the cooker and place on a cooling rack, or a trivet at the table. Let cool for at least 10 minutes before serving with your favorite ice cream, whipped topping, or vanilla yogurt.