Add the oil to a roasting tray and heat it in the oven for 5 minutes. Then add the beetroot, plums and vinegar to the roasting tray. Season with salt and roast for 30 minutes. Leaving a little of the stalk in place, slice the radicchio into thin wedges. Add it to the beetroot tray with the chilli, maple syrup or honey and a bit more salt, and give it all a good stir. Roast for a further 10 minutes. Add the garlic and stir it through the contents of the tray, then remove the tray from the oven.