Arrange the tomatoes over the bottom of 4 lightly buttered individual ovenproof dishes or 1 large ovenproof dish. Cut the fish into chunks and scatter them over the tomatoes. Top with the grated cheeses and chopped chives.
Pour over the cream, place on a baking sheet and cook in a preheated oven, 220°C (425°F), Gas Mark 7, for about 20 minutes until the gratin is bubbling and golden and the fish is cooked.
Serve immediately with steamed new potatoes, if liked.