Pre-heat the oven to 170°C, 325°F, Gas Mark 3
Melt the butter in a saucepan, add the flour and stir quickly to make a roux. Gradually add the milk, stirring continously to remove any lumps. Bring to the boil, then reduce the heat and simmer for 1-2 minutes, until thickened slightly.
Add the Basil and season to taste.
Remove from the heat and add the tomato purée, egg yolks and cheese, mix well.
Whisk the egg whites with a pinch a salt, until stiff. Delicately fold them into the yolk mixture.
Grease 4 individual ramekins and fill ¾ of the way up. Place the ramekins on a baking tray and cook in the oven for 15 minutes, until risen. Serve immediately, with a salad on the side.