Tomato Sauce

• 1 onion, peeled and sliced
• 25 g (1 oz) unsalted butter

Gently cook the onion in the butter until translucent and soft. Add the charred tomatoes and white wine. Bring to the boil, and simmer for 10 to 15 minutes. Rub through a sieve, season lightly, and stir into the hot fondue just before serving.