- Roll out pastry to 3mm thick and use to line a 24cm deep-sided tart tin. Prick the base with a fork and chill for at least 30 minutes.
- Heat a saucepan, add oil and onion and cook over a moderate heat for 5 minutes to soften but not colour.
- Add tomatoes and simmer for 20 minutes, stirring regularly until the mixture thickens and much of the moisture has evaporated. Remove to cool.
- Heat the oven to 220 degC. Line pastry case with baking paper or foil and fill with baking beans to blind bake for 15 minutes.
- Remove paper and beans and return pastry case to the oven for a further 5 minutes to dry out. Reduce oven temperature to 160 degC.
- Mix mustard, eggs, cream, grated cheese and sage into cooled tomato mixture and season well with salt and pepper.
- Pour mixture into the pastry case and bake for 30 minutes or until just set. Slice to serve.