Heat your oven to 180°C/Fan 160°C/Gas 4. Trim the leek. Slice a cross into the top so you can open it out. Rinse to remove any grit. Finely slice the leek.
Finely slice the mushrooms. Finely grate the zest from the lemon. Juice it. Fill a kettle and boil it.
Warm 1 tbsp olive 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 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 850ml hot water from the kettle. Stir to mix.
Stir the risotto rice into the pan with the lemon zest and thyme sprigs. Pour in all the stock. Gently cook and stir over a medium heat for 5 mins.
Spoon the wet risotto into a heatproof dish. Cover with a lid or some foil. Pop in the oven. Bake for 30 mins till almost all the stock has been absorbed. Take the lid or foil off. Cook for a further 5 mins to dry it out a little.
While the risotto cooks, tear the leaves off half the lettuce. Pop in a bowl. Whisk half the lemon juice with 1 tbsp olive oil and some seasoning to make a dressing. Pour over the lettuce. Toss to mix.
Finely grate the Parmesan. Stir into the risotto with the remaining lemon juice. Fish out the thyme sprigs. Taste and adjust the seasoning. Serve the risotto with the dressed lettuce.