This was a pantry recipe that came out amazing.  Preheat the oven to 375 degrees.
Season boneless, skinless chicken thighs with salt and pepper.  Brown each side of the thighs in a skillet on medium high heat using a few tablespoons of olive oil.  The thighs will not be cooked through, this step is just to brown them.  Transfer thighs to a single layer in a baking dish.  
 
In the same skillet add thinly sliced onions, a tablespoonful of butter and olive oil.  Saute the onions on medium low to medium heat until carmelized, stirring occasionally.  
 
While the onions saute, spread a thin layer of your favorite mustard on each chicken thigh.  If you love spice, using a sweet, spicy mustard here would be delicious.  
Cut four apples into thin slices and place in an even layer over the chicken.  To peel or not is up to you.  Use your favorite apple(s) for cooking.  
 
Once the onions are carmelized, spread them in an even layer over the apples.  Cover the baking dish with foil and bake for 25 minutes or until apples are softened and chicken is cooked through.  The sauce this makes is delicious to be served over a grain or sauteed greens.  The greens can be cooked in the same skillet while the chicken bakes.
 
This recipe can easily be adapted to pork chops or chicken breasts.
If the apple onion topping is too sweet for your taste, add a bit of apple cider vinegar to the onions near the end of their cooking time.  This will add a little more twang.  You can also add some red pepper flakes or a chopped jalapeno when cooking the onions.
 
Vegetarian/Vegan:
The topping would be delicious as part of a grain bowl, on a bean burger, or seared tofu and could be cooked just in the skillet with olive oil.   I have stirred it into cooked quinoa and chickpeas and added green onions.  It was tasty!