In a slow cooker, layer the ingredients as follows: onions, apples, then pork chops, sprinkling sage between each layer and on top.
In a small bowl, mix the mustard and cider and pour in slow cooker. Cook on low for about 8 hours. Remove pork chops and serve with the apples and onions on top.
*Variations: Use Fresh Poultry Seasoning Mix in place of the Sage.