Spray the inside of a slow cooker with nonstick spray. Combine the rice, dried apples, wheat berries, oats and barley in the crock. Stir in the water, maple syrup, vanilla, cinnamon and salt. Cover. (If making ahead, reserve in the crock overnight).
Set the slow cooker on high and cook for 2-1/2 hours until grains are tender but not mushy. Add the butter; stirring the muesli and adjust the texture by adding extra water to taste. Serve with yogurt and additional honey or maple syrup.
Makes 8 servings.
Tip: Create a self-serve muesli topping bar where your family and guests can customize their portion.