Preheat the oven to 220°C. Sift the ﬂour and a pinch of salt into a bowl. Rub in the butter until the mixture resembles ﬁne breadcrumbs. Mix in enough water to make a ﬁrm dough. Roll out the dough on a lightly ﬂoured surface to an oblong a bit larger than a 28 x 18 x 4 cm loose-bottomed tart tin. Line the tin with the dough and trim the edges, leaving about 1 cm above the top of the tin. Fold over the excess dough and press lightly with your fingertips to make a rim. Line the pastry case with crumpled greaseproof paper, ﬁll with baking beans and bake blind for 10 minutes.
Meanwhile, heat the oil in a small pan and cook the chopped onions on low heat for about 10 minutes, or until soft and golden brown. Take the tart case out of the oven, remove the paper and baking beans and reduce the oven temperature to 180°C.
Spread the onions evenly over the bottom of the pastry case. Combine the four cheeses and the snipped chives and spread over the onions. Beat the eggs and milk together and season with salt and pepper. Pour the liquid over the cheese and chive mixture and decorate the top with tomato slices.
Bake the filled tart in the centre of the oven for 35–40 minutes, or until the pastry is golden brown and the ﬁlling is lightly set. Serve the tart warm, cut in squares.