Put the potatoes in a small pan of boiling salted water and boil for 10-12 minutes, or until tender, then drain.
Meanwhile, dust the sole fillets in seasoned flour and shake off the excess. Heat the groundnut oil in a large, heavy-based frying pan over a medium heat, add the sole skin-side down and fry for 2 minutes. Carefully flip the fish using a palette knife and fry for another 1-2 minutes.
Remove the pan from the heat, add the butter and swirl it around the pan. Add the capers, parsley and a generous squeeze of lemon juice, and season well. Spoon the buttery sauce over the fish a few times to baste it, then divide the fillets between 2 serving plates and pour over the rest of the sauce.
For the potatoes, melt the butter and add the cooked potatoes and the watercress. Season and toss to wilt the watercress slightly. Divide the potatoes between the 2 plates, alongside the fish, and serve immediately (with some lemon wedges, if you like).