Heat a large heavy bottom skillet over medium high-heat. Add the ribs to the skillet and sprinkle with salt + pepper. Sear the ribs on all sides for about 1 minute per side. Remove the ribs from the skillet.
Arrange the onions on the bottom of your crockpot and sprinkle the brown sugar over them. Add the seared short ribs, garlic, mushrooms, apple cider and thyme.
Cover the crockpot and cook on low for 7-8 hours or on high for 5-6 hours. During the last 1-2 hours of cooking, add the apple slices, tossing to combine.
Once the short ribs are done cooking and easily fall apart, skim off any fat from the top of the crockpot. Lightly shred the ribs and remove and discard the bones. Keep the rib meat in the sauce, covered on low heat until ready to eat.
About 40 minutes before you’re ready to eat, make the potatoes. In a large pot of cold water, bring the potatoes and garlic to a boil. Salt the water and cook until the potatoes are tender, about 20 to 30 minutes.
Drain the potatoes, return the potatoes to the pot and mash over low heat, or mash in the bowl of stand mixer fitted with the paddle attachment, adding the cream, cheddar cheese, and 2 teaspoons salt.
In a skillet or small sauce pot, melt the butter over medium heat until just browned. The butter will melt, foam and froth, then begin to brown along the bottom. Whisk browned bits off of the bottom of the pan. Stir in the sage and let cook 30 seconds. Pour the browned butter into the warm mashed potatoes. Taste and season with salt and pepper if desired.
Divide the mashed potatoes between plates and top with the rib meat. Enjoy!