Preheat the oven to 180C/350F/Gas No 4. Lightly grease 6 x 8cm x 4cm deep/ 3″ x 1 ½” deep ramekin dishes. Line the bottom of the dishes with a circle of greaseproof paper.
Gently cook the onions for 10 minutes until translucent, add the sugar & turn the heat up to caramelize the onions. Keep to one side.
Melt the butter, stir in the flour to make a smooth paste & cook for 1 minute. Gradually stir in the milk, bring to the boil then reduce the heat to simmer & cook for a further minute. Beat in one egg yolk at a time; add 100g/ 6oz of cheese, mustard, salt & pepper.
Beat the egg whites in a clean bowl until stiff, stir into the cheese mixture.
Divide the onions between the ramekin dishes, then spoon the mixture on top of the onions. Bake for 20 minutes. Leave to cool, when cool remove the soufflés by running around bladed knife around the edge of each dish.
Place the soufflés onto a baking sheet & sprinkle the remaining cheese over the soufflés. Bake for 15 minutes until golden brown.
Serve with watercress salad. Makes a good light lunch with crispy bread or a starter.