Preheat oven to 200 C / Gas 6. Grease a 23cm (9 in) tart tin.
For the pastry:
Place flour in a bowl and rub in butter and lard until the mixture resembles breadcrumbs, add a little cold water to bind. Knead together and turn on to a lightly floured surface. Form into a ball then roll out pastry to line the prepared tin.
Press the pastry into the prepared tin and cover with baking parchment. Fill the pastry shell with dried beans.
Bake blind for 10 minutes on the centre shelf, then remove lining and beans and cook 15 minutes more. Remove from the oven.
For the filling:
Heat the 25g butter and oil in a frying pan and add onion. Cook and stir until soft, for about 15 minutes. Remove from heat and stir in tomato purée, salt and pepper and parsley.
Melt 40g butter in a small saucepan and stir in 40g flour. Cook and stir for 1 minute and add the milk gradually, whisking constantly to form a smooth sauce. Stir in 30g (1 oz) of grated cheese into the sauce. Add mustard and mix well. Remove from heat.
Spoon onion filling into the prepared pastry case and pour in the cheese sauce. Lay sliced tomatoes on top, season, and sprinkle in the remaining cheese.
Place tart under oven grill, and cook until cheese has melted.
This recipe calls for well flavoured cheese. In this instance any well aged cheese would work great. Depending on your taste, you could use a blue cheese such as Stilton in this recipe.