1. Butter (grease) soufflé dish and coat lightly with first quantity of grated Gruyère cheese. (This helps the egg (soufflé) mixture to cling to the sides of the dish as it rises. Dried breadcrumbs may be used instead of cheese.) Tie a band of doubled waxed paper, reaching about 5-7 cm above rim, around the circumference of the dish. Secure with string. (this helps the mixture to move up the sides of the dish.) Chill the dish well in the fridge. Preheat the oven to 190 ºC.
2. Blend cornflour and water in a bowl until smooth. Melt butter in a large heavy-based saucepan on top of the stove and stir in flour until smooth and a paste forms (this is called a roux). Stir briskly over low heat for 1 minute, but do not allow to brown. Remove saucepan from heat (stove). Stir cornflour mixture into roux, then milk, return to stove and heat over low heat, beating well with a wooden spoon or balloon whisk until mixture is smooth and heated through.
3. Remove from heat and add remaining grated Gruyère cheese. Beat until mixture is smooth and cheese has melted. Season with salt, pepper and nutmeg. Return to heat. Beat egg yolks in briskly, one by one, until mixture is soft and creamy. Cook for 1 to 2 seconds longer and remove from heat. Cool slightly.
4. Whisk egg whites until foamy, using a balloon whisk. Add cream of tartar and beat until stiff but not dry. Using a spatula, fold a quarter of egg whites into just warm cheese and yolk mixture. Gently fold cheese and yolk mixture back into remaining whisked egg whites, as described in tips.
5. Spoon soufflé mixture into prepared dish. Smooth surface with a spatula, then make a slight depression in the centre. Place on centre rack of preheated oven and bake for about 30 minutes, or until puffed up and golden.
6. TOMATO AND CORIANDER SAUCE: While soufflé is baking, heat olive oil in a saucepan. Add diced tomatoes, seasonings, tomato paste and water by the spoonful, cover and simmer over moderate heat until tomatoes have softened. Stir in coriander and continue simmering for 2 to 3 minutes longer.
7. TO SERVE: Remove soufflé from oven, whip off paper collar and serve immediately in heated wide-brimmed soup plates, with tomato and coriander sauce spooned around.