Preheat oven to 375 degrees F.  Wash apples and pat dry.  Core apples, only going about half way through.  You want to remove the seeds and create a well, but not go all the way to the bottom.  I use an apple coring tool and scoop the rest out with a small spoon (or melon baller).  There should be about an inch of space left in the bottom.  Melt butter in the microwave for 20 seconds.  Stir maple syrup into the butter until thoroughly mixed.  Place cored apples into a small baking dish and pour butter mixture, evenly, into the center of each apple.  Pour apple juice or cider into the bottom of the pan (juice should be enough to cover bottom of pan) and bake, uncovered, for 1 hour until apples are soft.  Carefully place apple into a small bowl and slice in half, being careful not to spill the maple butter filling.  Top each apple with ¼ cup Greek yogurt and sprinkle with toasted nuts.  Drizzle a little extra syrup on top, if desired.