Score a criss-cross pattern on the fat side of the rack. Season and place in a bowl with the garlic, half the thyme and some seasoning. Pour a couple tbsp of the port over and marinate a couple of hours or overnight preferably.
To prepare, pat the meat dry and sear in a very hot frying pan. Preheat oven to 200°C.
Rub the mustard all over the lamb and sprinkle with more seasoning and the remaining thyme. Roast for 15 minutes for rare, 20 for medium and 25 for well done. Remove, cover with foil and let sit for 10 minutes.
Heat the port until boiling and add the cherries. Remove from heat and let sit for 5 minutes.
Scrape out most of the oil from the lamb roasting tin, keeping 2tbsp. Heat the pan on the hob, add the shallots and saute for the few minutes before adding the flour. Whisk for 1 minute over medium heat and then add the port/cherries and stock. Simmer until the sauce thickens, about 5 minutes. Add the jelly and season with salt and pepper. Remove from heat.
Slice the lamb and serve with sauce and celeriac/potato mash.
Celeriac and potato mash with chives
Peel and chop the veg into 2.5 cm (1 inch) chunks. Place the vegetables in a large saucepan of cold salted water and bring to the boil. Cook for 15 minutes or until tender, then drain.
Return to pan and heat until all of the watery liquid has disappeared. This is important or the texture will be slushy instead of thick.
Add the butter and roughly mash until combined.Season with salt and pepper, add the milk and mash again until fluffy and light.
Just before serving, gently reheat and then add the chives to combine.