Melt the butter in a heavy-based saucepan, add the garlic, onion and tomatoes and cook over a low heat, stirring occasionally, for 3–4 minutes. Add the oregano and olives and season to taste with salt and pepper. Remove the pan from the heat and leave to cool.
Meanwhile, for the soufflé, melt the butter in a saucepan, add the flour and cook, stirring constantly, for 1 minute. Remove the pan from the heat and gradually add the cream or milk, stirring vigorously after each addition to ensure that it is fully incorporated. Return the pan to the heat and bring to the boil, stirring constantly, until thickened. Remove from the heat and beat in the egg yolks, 1 at a time. Add the cheese and stir until it has completely melted. Remove from the heat and leave to cool.
Whisk the egg whites in a large bowl until just stiff enough to stand in peaks. Mix about 2 tablespoons of the egg whites into the cheese mixture, then carefully fold in the remaining egg whites with a metal spoon.
Grease a 1.5 litre (2½ pint) soufflé dish and place it on a baking sheet. Spread the cooled tomato mixture in the dish and cover with the soufflé mixture. Bake immediately in a preheated oven, 190°C (375°F), Gas Mark 5, for 35–40 minutes until well risen and golden brown. Serve immediately.