Using a sharp knife, remove the sinew from the beef cheeks. Peel and halve the shallots and peel and finely slice the garlic.
Place half the butter in a medium ovenproof casserole dish over a medium heat. Sear the meat all over, until browned on all sides, then leave to one side.
Add the shallots and fry for 8 to 10 minutes, or until slightly browned. Stir in the garlic and pick the thyme leaves straight into the pan. Fry for a couple of minutes, then return the beef cheeks to the pan and pour in the red wine.
Bring to the boil, cook the wine for around 5 minutes, until reduced by half, then finely grate in the chocolate (if using).
Pour in enough beef stock to cover, and turn up the heat. Bring back to the boil, season, then reduce the heat to the lowest setting. Cover with a tight layer of tinfoil and a lid, then place the pan in the oven and slowly braise for 4 to 4½ hours, or until incredibly tender.
When the beef is almost ready, make the creamed parsnips. Bring a large pan of salted water to the boil.
Peel and chop the parsnip, celeriac and apples into 3cm chunks, taking care to remove the apple seeds and core. Add the fruit and veg to the pan, and simmer for about 12 minutes, or until cooked through. Drain and steam dry over the hot pan.
Stir in the rest of the butter and then spoon into a food processor. Pour in the milk and add a good pinch of sea salt and black pepper, then blitz until smooth, adding a little more milk if needed (you can do this in the pan with a hand blender if you prefer).
Remove the cheeks from the casserole dish, then place the dish over a high heat. Cook rapidly until the sauce thickens. Stir in the jam, taste and adjust the seasoning, then add the cheeks back to the pan and baste.
Serve with the creamy parsnips and all your favourite trimmings.