Place 1/3 cup (80ml) of oil in a food processor with the honey, rosemary, Dijon mustard, fennel seeds, 1 tbs sea salt flakes and 1 tsp pepper. Process for 30 secs or until finely chopped. Place lamb on a chopping board. Score fat side of the lamb in a diamond pattern, being careful not to cut deeply into the meat. Rub rosemary mixture over lamb. Line a roasting pan with foil and add lamb. Set aside at room temperature for 1 hour to develop the flavours.
Preheat oven to 150°C (130°C fan-forced). Roast lamb for 11/2-13/4 hours or until an instant-read thermometer inserted into thickest part of lamb reads 49°C. Remove from oven. Increase oven to 250°C (230°C fan-forced).
Roast lamb for a further 10-15 mins or until browned and internal temperature of lamb registers 54°C for medium-rare doneness. Rest for 30 mins.
While the lamb is resting, in a large saucepan, add potatoes and enough water to cover by 2cm. Add 2 tbs of the vinegar to pan and season with salt. Bring to boil over medium-high heat. Reduce heat to medium. Simmer for 12 mins or until potatoes are tender. Reserve 1/4 cup (60ml) cooking liquid. Drain potatoes, being careful not to break them up. Spread over a rimmed baking tray. Set aside for 5 mins to cool.
Whisk remaining oil and vinegar in a bowl with shallots, parsley, capers, dill, wholegrain mustard, 2 tsp sea salt flakes, 1 tsp pepper and reserved cooking liquid. Add potatoes. Gently toss to coat.
Carve the lamb into 1cm-thick slices. Serve with the warm potato salad.