Place a wide-based casserole or sauté pan, with a lid, on a medium heat and add one-third of the olive oil. Once hot add the leeks, cut side down, and fry for 2 minutes each side, until slightly golden. Now add the diced carrot and onion and cook for a further 5-6 minutes with the thyme springs and most of the chopped garlic. Reserve a little less than 1/2 a clove for the dressing (to serve 4).

Pour in the hot vegetable stock along with the lentils and bring to a simmer. Season with salt and pepper before then covering with a lid and cooking on a low heat for 30 minutes, turning the leeks halfway way through.

To make the dressing, finely chop or grate the remaining garlic and mix together with the dijon mustard, red wine vinegar and remaining olive oil. Season to taste with salt and pepper. You can do this in small bowl, or a jar with a tight-fitting lid and simply giving the ingredients a good shake.

Once the lentils are just about cooked, Increase the heat and cook, with the lid off, until most of the stock has been absorbed - roughly another 5 minutes. Remove from the heat and lift out the sprigs of thyme. Pour over the dressing, add the chopped dill and mix well. Allow to cool slightly, before dividing the watercress between the plates and spooning over the lentils and leeks. Finish with a grating of lemon zest (if using) before tucking in.