1. Warm a deep frying pan or casserole dish over a medium heat, then add 1 tbsp olive oil and the diced chicken leg. Fry, turning a few times, for 5-6 mins till the chicken is golden brownr.
2. While the chicken fries, trim the roots and woody greens off the leeks. Halve them, rinse out any grit, then finely slice them. Trim the dry tips off the fennel bulb, then finely slice it, setting any feathery fronds aside for later. Peel and grate or crush the garlic.
3. Add the leeks, fennel and garlic to the pan with the chicken with 2 tbsp cold water. Season with a pinch of salt and pepper, stir and cover with a lid (or use a baking tray if you don’t have a lid). Fry, stirring often, for 10 mins till the veg have softened and started to brown. If they start to catch or the pan starts to dry out, turn the heat down and add another splash of water.
4. Preheat your grill to high. Finely chop the chives. Stir half the chives and 1 tbsp wholegrain mustard into the pan. Pour in 150ml water, stir, pop the lid back on and simmer for 15 mins, stirring now and then. The chicken should be cooked through and the veg very tender.
5. Take the pan off the heat and add half the pot of half fat crème fraîche. Gently stir to mix. Taste the sauce and add more salt or pepper, if you think it needs it. If your pan can go under the grill, slide under the grill for 5 mins. If not, transfer the chicken, veg and sauce to an ovenproof dish and grill. After 5 mins, scatter over the breadcrumbs if you're using them, and grill for 1-2 mins till golden brown and bubbling.
6. While the gratin cooks, measure out ½ tbsp cider vinegar and whisk it with ½ tbsp olive oil and some salt and pepper to make a dressing. Toss the salad leaves with the dressing. Serve the chicken, leek & fennel gratin with the salad leaves on the side.