Place the ham in your slow-cooker, flat-side down.
Sprinkle the brown sugar all over the ham and use your clean hands to work the sugar into the ham. Pour the maple syrup and apple cider over the top of the ham.
Place the lid on the slow-cooker and turn on LOW and cook for 2 to 3 hours. (no less than 2, no more than 3).
Take the lid off and use a large spoon to pour the cooking liquid all over the top of the ham. Put the lid back on and slow-cook for another 30 minutes.
Use a couple of large spoons or spatulas to carefully remove the ham from the slow cooker and place it on a cutting board or serving platter. Cover with foil if not serving within 15 minutes.
To make the sauce, use a measuring cup to remove 2 cups of the cooking liquid from the slow-cooker. In another medium-sized bowl, mix together the two mustards and the apple cider vinegar. Stir in the reserved liquid. Whisk until fully combined. Taste, it may need a little more salt, if so, sprinkle some in. Transfer to a gravy boat and serve with sliced ham.
Go with boneless or bone-in, but we prefer bone-in. Be sure to not let the ham cook for too long. 3 hours is about the max you want to go. To keep the ham warm, turn the setting to WARM after the braising period is done. We like to pull the slices aways from the bone and arrange them on a platter to make it easy for guests to serve themselves. We also pour a little of the sauce over the ham.You can also cook cut carrots and/or parsnips in with the ham, but you'll need to increase the time to about 3 to 3 1/2 hours. Be careful not to overcook!