Put drained yams, and peeled and sliced apples in a greased 9x13 baking dish.
Add sugar, brown sugar, salt, butter and boiling water in a medium sauce pan and melt together.
In separate cup combine cornstarch and cold water until well combined and then add to the sugar mixture. Cook on medium high heat until clear and bubbly. Pour caramel mixture over yams and apples and bake at 350 for 20 minutes.
Sprinkle yam casserole with cinnamon, nutmeg and marshmallows and bake an additional 5-10 minutes or until marshmallows are melted and golden brown.
Video
Notes
You may not need all of the syrup mixture. Start pouring over your apples and yams and stop when you feel that everything has been well coated but it's not too close to the rim of the pan. Store leftovers in the fridge in an airtight container for 5-7 days.