Preheat oven to 180C.
Crush the fennel seeds with the salt using a pestle and mortar. Put in a large bowl with 1 tbsp of the oil and add the potatoes. Mix well and tip into a large roasting dish. Cover with foil and cook in the oven for 20 minutes.
Season each salmon fillet and wrap a slice of prosciutto around each one. Set aside.
Remove the foil from the potatoes and continue cooking for 10-15 minutes until almost tender. Arrange the tomatoes and salmon on top of the potatoes and sprinkle the fish with the remaining oil. Return to the oven and cook for 15-20 minutes until the salmon is done.
Meanwhile, mix the mayonnaise, yogurt and lemon juice together in a bowl. Add the watercress and capers and mix again. Add salt and pepper to taste.
Serve the salmon and potatoes with the watercress mayo and wedges of lemon.