Pre-heat the oven 180°C/160°C Fan/Gas 4. Line a large oven tray with foil, leaving enough overhang that you will be able to wrap the foil over the top and the edges will meet in the middle to seal.
Place the beetroot, half the garlic and half the thyme, along with all the sugar and salt in the foil-lined tray, and add 100ml of the red wine vinegar. Close the foil to form a pillow and then pour 500ml of water into the tray, around the outside of the foil.
Bake the beetroot for 40 minutes, until a knife goes easily through the largest beetroot.
While the beetroot are cooking, make the vinaigrette. Mix the oil, remaining vinegar and mustard together, then season well with pepper and leave to one side.
When the beetroot are ready, allow them to cool slightly, then remove them from the tray while still warm. Using a small knife, peel away the outer skins and cut each beetroot into quarters. Add the beetroot pieces to the vinaigrette and leave them to marinate for 2 minutes.
Cook the lentils until al dente (about 30 minutes) or according to the packet instructions, adding the remaining garlic and thyme to the cooking water. Allow the lentils cool a little in the cooking water, then drain them, discarding the thyme and garlic. Toss the lentils into the beetroot, then add the flat-leaf parsley and toss again. Serve warm.