In a large roasting tin, toss the potatoes with 1 tbsp oil and some seasoning. Roast for 20 minutes.
Heat 1/2 tbsp oil in a pan. Add the onion and cook for 5 minutes, or until softened. Add the garlic and fennel with a little seasoning, and cook for 2 minutes more.
Spoon the amixture over the potatoes. Top with the tomatoes, dill and chilli, then return to the oven for 5 minutes. Arrange the plaice fillets over the veg mixture. Squeeze over half the lemon, then drizzle with the remaining oil; season well. Cut 4 lemon slices from the remaining half and sit two on each fillet. Return to the oven for 8 minutes, or until the fish is cooked through and the veg is tender.