Place the potatoes in a saucepan, cover with water and add a pinch of salt. Bring to the boil then simmer for 12-15 minutes until tender. Drain and set aside.
Meanwhile, to make the sauce, combine the sugar, mustards, vinegar and ¼ tsp salt in a serving bowl. Stir in 50ml of water then gradually whisk in the oil until well-combined. Set aside.
Heat the oven to gas 7, 220°C, fan 200°C. Brush a large rectangle of double-thickness foil with oil and spread out, oiled side up, on a roasting tin large enough to hold the salmon. Trim the fennel bulbs, then halve from top to bottom and slice very finely. Reserve any fronds. Coat the fennel slices with 2 tbsp of oil, season and spread out over the tin.
Make 3 or 4 shallow slashes on each side of the fish and season inside and out. Rub the skin with olive oil and stuff the cavity with lemon slices and the reserved fennel fronds. Wrap the bacon rashers around the fish in a criss-cross pattern, tucking the cut ends underneath.
Put the fish on top of the fennel, bring the edges of the foil up and pour in the lemon juice plus 100ml of water. Scrunch the edges of the foil together to form an air-filled parcel, then cook for 25 minutes.
Tip the drained potatoes onto a large baking tray and use the base of a cup to gently squash them. Drizzle with olive oil, season and toss with the lemon zest and half the chopped dill. Set aside.
After 25 minutes, remove the fish from the oven and carefully open the foil, pushing it down to expose the fish and fennel. Return to the oven to brown for 10-15 minutes. Put the potatoes in the oven at the same time (the top shelf is best); they will need 20-25 minutes.
After 10-15 minutes, check that the fish is thoroughly cooked through, then remove it from the oven and cover loosely with foil. Set aside for 10 minutes to rest. When the potatoes are golden and crisp at the edges, remove them from the oven and toss with the remaining dill.
Carefully transfer the salmon and fennel to a warmed serving platter. Serve with the hot potatoes and the sweet mustard sauce.
For top tips on protecting you and your family when preparing raw meat and poultry, visit Food Safety in the Home.