To make the pastry, sift flour and salt together, add butter and rub in until mixture resembles fine breadcrumbs. Mix to a firm dough with egg yolk and 4-5 tsp cold water. Wrap and chill for 30 minutes. For the filling, melt butter in a large pan and fry onions for 10-12 minutes until soft. Leave to cool. Preheat oven to 200°C, Gas 6. Roll out pastry on a lightly floured surface and use to line a 23cm fluted flan tin. Line pastry with greaseproof paper, add baking beans and bake blind for 15 minutes. Lower oven temperature to 180°C, Gas 4. Remove the baking beans and paper and return pastry case to the oven for 5 minutes. Raise oven temperature back to 200°C, Gas 6. Mix together the cooked onions, flour, eggs, cheese, cream and seasoning. Add to the pastry case and cook for 25-30 minutes. If you want to serve the tart warm, it will taste best if you allow it to stand for 15 minutes after removing from oven.