Put the potatoes in a saucepan of lightly salted cold water, bring to the boil and cook for 15—20 minutes or until just cooked through. Drain the potatoes and leave them to cool.
Reserving the fronds for garnish, cut the fennel into thin wedges, leaving the root end intact to hold the pieces together. Put the fennel in a baking tin, drizzle with 1 tablespoon of the oil, season with salt and pepper and cook in a preheated oven, 190°C (375°F), Gas Mark 5, for 15 minutes until roasted and golden.
Meanwhile, cut the potatoes in half. Heat 1 tablespoon of the oil in a frying pan and fry the potatoes until they are crisp and golden. Drain on kitchen paper and transfer to a large salad bowl.
Make the dressing. Mix all the ingredients in a bowl.
Heat the remaining oil in a large frying pan until it is smoking and carefully fry the squid for 2—3 minutes. Add the squid to the bowl. Remove the fennel from the oven, leave to cool slightly and add to the bowl with the dressing and combine gently. Stir through the watercress and serve at once.