1. Slice the potatoes into ½cm-thick rounds (no need to peel). Place in a large pan and cover with cold water. Add a pinch of salt and pop on a lid. Bring to the boil and cook for 8-12 mins, till the potatoes are tender. When cooked, drain and set to one side.
2. Meanwhile, warm a frying pan (ovenproof if possible) on a medium heat. Add ½ tbsp oil and the diced chicken leg. Fry for 6-8 mins, turning once or twice, till golden and cooked through.
3. While the chicken cooks, trim the roots and top 3cm off the leeks. Halve lengthways, rinse out any grit and thinly slice the leeks. Peel and grate or crush the garlic. Pick the thyme leaves off their sprigs (if the sprigs are tender and white, you can keep the leaves on them). Fill and boil your kettle.
4. Lift the chicken out of the pan and set aside on a plate. Add the leeks to the pan with 4 tbsp cold water and a pinch of salt. Gently cook for 8 mins, stirring often, till starting to soften. Add the garlic, thyme and ½ tsp smoked paprika. Cook and stir for 1 min.
5. Pour 250ml boiling water into the pan. Crumble in the stock cube and add the chicken back to the pan. Stir, pop on a lid and simmer for 5 mins. Meanwhile, grate the cheddar. Warm your grill to a high setting.
6. Pour half the pot of double cream into the pan. Add half the grated cheddar and plenty of salt and pepper. Add the sorrel and stir to just wilt it. If your pan is ovenproof, add the potatoes and fold everything together. If not, transfer the contents of the pan to an ovenproof dish, add the potatoes and fold through.
7. Scatter over the remaining cheddar and crack over plenty of black pepper. Slide under the grill for 8-10 mins, till the top is golden and bubbling.
8. While the gratin is cooking, make the salad. Add ½ tbsp mustard to a large bowl and whisk with 2 tsp olive oil and a little salt and pepper. Thinly slice the tomatoes and add them to the bowl. Roughly tear the lettuce leaves, add them to the bowl and toss to dress.
9. Serve the creamy gratin on a couple of warm plates, with the mustardy salad on the side.