Heat your oven to 180°C/Fan 160°C/Gas 4. Trim the top and bottom off the leek. Halve it and rinse out any grit. Finely slice it. Peel and finely chop the onion. Finely slice the mushrooms. Grate the zest from the lemon. Put the lemon to one side for later.
Warm ½ tbsp oil in a large pan. Add the diced chicken leg. Stir and fry over a medium heat for 5 mins till the chicken is golden all over.
Add the leek, onion and mushrooms to the pan. Season. Fry for 5 mins till the veg are soft and juicy. Pop the stock cube into a heatproof measuring jug. Pour in 1 ltr boiling water. Stir to mix.
Stir the risotto rice into the pan with the lemon zest and thyme sprigs. Pour in all of the stock. Gently cook and stir over a medium heat for 5 mins.
Spoon the risotto into an ovenproof dish. Cover with a lid or foil. Bake in the oven for 30 mins till almost all the stock is absorbed. Take the lid or foil off. Cook for a further 5 mins to dry it out a little.
While the risotto cooks, squeeze the juice from half the lemon into a small bowl. Add ½ tbsp oil and some seasoning. Whisk with a fork to make a dressing. Toss the salad leaves with the dressing.
Finely grate the Parmesan. Stir most of it into the risotto with a squeeze of juice from the remaining lemon half. Fish out the thyme. Taste and adjust the seasoning. Serve the risotto in bowls topped with the remaining Parmesan and the salad on the side.