Preheat the oven to 200°C, 400°F, Gas 6.
Halve and core the fennel bulbs. Thickly slice and divide between two foil parcels with the olive oil, a squeeze of lemon juice and salt and pepper. Bake in the oven for about 25 minutes until completely tender.
Meanwhile cook the potatoes in boiling salted water until tender – about 18-20 minutes. Test with a knife to ensure they are properly cooked, then drain and leave to cool a little. Halve the potatoes if necessary, then toss while still warm with the mustard and extra virgin olive oil in a salad bowl.
Slice the salmon into strips and add to the potatoes with the capers and dill. Add the fennel slices, then check to see if the salad needs more lemon juice and seasoning. Serve with a teaspoon of sour cream on each portion if desired.