Halve the leek. Rinse out any grit and thickly slice it. Roughly chop the celery. Peel and dice the carrot. Slice or quarter the mushrooms, depending on how big they are. Peel and grate or crush the garlic. Rinse the lentils in a sieve.
Heat 1 tbsp oil in a large pan. Add the chicken. Fry for 5 mins till the chicken is browned all over. Lift out of the pan. Set aside.
Add the leek, celery, carrot and mushrooms. Season. Cover and fry the veg over a medium-low heat for 7-8 mins till they’re soft. Stir every now and then. While the veg cook, tear the chicken into shreds with 2 forks.
Take the lid off the pan. Add most of the thyme and all the garlic. Turn the heat up a little. Season with a little salt and pepper. Cook for 2 mins, stirring once or twice.
Stir the lentils into the pan. Add the shredded chicken. Crumble in the stock cube. Pour in 1 ltr boiling water. Cover. Simmer for 15-20 mins till the lentils are tender. If the stew seems too thick, add a splash more water.
While the stew simmers, pull the kale leaves off the thick middle cores. Finely chop the leaves.
Add the kale to the stew. Simmer, without the lid on, for 3-5 mins till the kale is tender.
Taste the stew and add more salt and pepper if you think it needs it. Ladle into warm bowls, top with the remaining thyme and serve straight away.