Season both sides of your pork chops with salt and pepper.
You have the option to sear your pork chops before placing them in your slow cooker. I rarely do. If you want to do it this is what I suggest. In a large skillet, over medium high heat, place 1 or 2 tablespoons of oil and heat until oil shimmers or a drop of water flicked in it dances. Brown pork chops on both sides. Use more oil if necessary to brown all of your pork chops. Set aside.
Prepare your slow cooker with cooking spray. Place onion slices and garlic across the bottom of your slow cooker. Place your pork chops on top of onions and garlic in slow cooker. If they don't all fit, place all that will fit.
In medium sized bowl, whisk together brown sugar and soy sauce. Next add mandarin oranges and their juice. Gently stir to until blended.
If your pork chops all fit in one layer in your slow cooker dump the mandarin mixture all over top of your pork chops and you are ready to cook them. If you have more pork chops that need to go in your slow cooker pour half of the mixture on top of your pork chops. Add a layer of your remaining pork chops and pour remaining sauce over top.
Cook on high for 3 hours or low for 5-6 hours or until the internal temperature of the pork reaches at least 150 degrees.
Remove your pork chops from your slow cooker as carefully as possible. I usually use a spatula. Remove as many mandarin oranges as possible using a slotted spoon and set aside on top of your pork chops.
Turn your slow cooker up to high and using a soup ladle place some of the juice into a small bowl with your cornstarch. Stir until all of the cornstarch is dissolved. Dump the dissolved cornstarch into your slow cooker and stir util it is combined. Allow to boil until liquid thickens. Return your pork chops and mandarins to the slow cooker for a few minutes to warm them up if desired. I usually don't.
Another, quicker option, is to pour the remaining liquid in your slow cooker, after you have added the cornstarch, into a pot and put it on the stove on medium high. Bring it to a boil, reduce heat to a simmer, and stir until it is thick enough for your personal taste.
Serve your pork chops covered with mandarins and sauce or gravy. and serve immediately. I usually serve rice or even mashed potatoes with this dish so I can put some gravy or sauce on top of it too! Enjoy!