Heat your oven to 200°C/Fan 180°C/Gas 6. Scrub the potatoes. Cut them into 1cm thick slices. Zest the lemon and set the zest aside. Cut half the lemon into quarters.
Line a baking tray with baking paper. Place the potatoes and lemon quarters on the tray. Drizzle over 1 tbsp oil. Scatter over 1 tsp fennel seeds and some salt and pepper. Toss together. Bake for 30 mins, tossing everything halfway through cooking. The potatoes should be golden and crisp.
While the potatoes cook, halve the tomatoes. Pop them in a bowl with the rocket.
Juice the remaining lemon half and whisk it with ½ tbsp oil and some salt and pepper. Toss with the rocket and tomatoes.
Meanwhile, place the butter in a small pan over a low heat. Add the saffron, lemon zest and a pinch of salt and pepper. Heat till the butter melts, then set aside. Chop the dill fronds, discarding the thick stalks.
Place a dressed crab on each plate. Mix the meat together in the shells with a fork to fluff it up. Pour half of the saffron butter over each one. Serve with the fennel roasted potatoes, rocket salad and wedges of roasted lemon. Scatter over the dill to garnish.