Preheat the oven to 225 degrees F.
In a heavy oven-proof pan, heat 3 tablespoons of the olive oil over a high heat.
Season the short ribs all over with the salt and pepper.
Sear the ribs on each side for about 1 minute, transfer to a plate.
Deglaze the pan with ¼ cup of cider.
Add the garlic and cook for about 30 seconds.
Add the mushrooms and cook for a further 3-4 minutes and the mushroom are getting soft.
Add the short ribs back into the pan and add the remaining cider, thyme sprigs and beef stock.
Bake for about 4 hours or until a sharp pointed knife can pierce the meat easily.
While the ribs are cooking, peel the sweet potatoes and half them.
Place them in a pot of boiling water and cook for about 30 minutes and they can be pierced easily.
Drain and add the butter, cream and Parmesan.
Using a potato masher, crush them until almost all the lumps are gone and the ingredients are well combined.
Once the ribs are ready the potatoes can be reheated over a low heat, stirring frequently.
Shred the meat from the bones and transfer to a plate.
Discard the thyme sprigs and stir the sauce.
Bring the sauce to a boil and boil for 3-4 minutes to thicken it a little.
Serve the meat over the sweet potatoes and pour some of the sauce on top.