What you need to do:

Set the oven to 220°C or 425°F. Melt the butter in a saucepan over a low heat, then mix the in plain flour and cook out for just 30 seconds. Add in the milk and mix until the sauce is smooth. Let it simmer for a few minutes – keep an eye on it so it doesn’t burn. Add one egg yolk and mix in quickly, then remove the saucepan from the heat. When the mixture has cooled, add the rest of the egg yolks and season. Add the Swiss cheese, basil and tomato puree.

Now for the egg whites. Whisk them in a bowl with a pinch of salt, using a Hand Mixer at a low speed at first, switching to medium-high to finish. Make sure you go in wide circles around the bowl to get lots of air in, until they are light and fluffy – a dipped in spoon should leave a trail. Add two dessert spoonfuls of the egg white mixture to the base mixture, and mix well. Gently fold in the remaining mixture as lightly as possible. Place mixture into the 4 buttered ramekins, filling them two thirds full so there is room for the soufflé to rise. Cook for 25-30 minutes in the oven, and serve immediately.

And another thing…

Adding a pinch of cream of tartar to the egg whites before you whisk them will stabilise the mixture – and help prevent your soufflé from deflating.