To make the layered potato cake, heat the oven to 180C/fan 160C/gas 4 and use a little of the duck fat to oil a round 30cm x 6cm deep ovenproof dish or pan. Heat the rest of the duck fat in a small pan until melted, then add the garlic and thyme. Tip into a large bowl with the potatoes and some seasoning, and toss well. Layer the potatoes in the ovenproof dish, then cover with foil and bake for 45 minutes. Remove the foil for the final 15 minutes to crisp up and colour the top (turn up the heat if needed). Reheat to serve.
Turn down the oven to 160C/fan 140C/gas 3. Heat the vegetable oil in a large, wide casserole and brown the duck for 2-3 minutes on all sides until golden, then remove to a plate. Pour in the red wine and scrape any sticky bits off the bottom. Remove from the heat and put the herbs in a layer on the bottom of the casserole.
Mix together the garlic, coriander seeds and lots of seasoning, then rub all over the duck. Put the duck breast-side up in the casserole, then pour in the stock. Roast for 1 hour and 30 minutes, then turn up the oven to 200C/fan 180C/gas 6 for 10-15 minutes or until the skin is crisp and brown. Put onto a platter, cover loosely with foil and rest for 15 minutes. At this point you could put the potato cake back into the oven to warm through. Carve and serve with the cooking and resting juices.
Heat a grill pan over a high heat. Toss the spring onions with a little rapeseed oil and some seasoning. Cook for 3-4 minutes, turning, until charred and tender.
For the salad dressing, whisk together the mustard, honey and vinegar in a bowl with a little seasoning. Slowly pour in the oil in a steady stream, while whisking, until combined. Tip the spring onions, shallots, radishes and watercress into a large bowl, and toss with enough dressing to lightly coat (the remaining dressing will keep in the fridge for a week). Serve with the duck and the potato cake.