Bake the potato wedges in a preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for 18-20 minutes. Wash the tomatoes and grate them dry. Clean 6 tomatoes and cut them into small pieces.
Wash, clean, quarter and seed 2 tomatoes. Cut the flesh into small cubes. Wash and clean the spinach and drain well. Wash the plaice fillets, dab dry and season with some salt and pepper.
Spread the skin side thinly with pesto, then cover with 3-4 spinach leaves and sprinkle with tomato cubes. Roll up the fillets firmly and pin them with a toothpick. Bring wine and 150 ml water to the boil, place the rolls inside.
Leave to stand at low heat for about 8 minutes. For the vinaigrette, peel and finely dice the onion. Mix vinegar and onion cubes, fold in oil. Season to taste with salt, pepper and sugar.
Mix vinaigrette, tomatoes and spinach. Arrange plaice rolls, potato wedges and salad on plates.