First make the pastry by sifting the flour, mustard powder into a mixing bowl, then stirring in the oil. Then add enough cold water to make a dough that leaves the bowl clean. Leave to rest for half an hour at room temperature.
Meanwhile, prep the filling. Put the oil in a heavy-based saucepan, then add the onions, stir to get them well coated in the oil, and cook them (uncovered) over a medium heat for about 10 minutes until they have reduced and turned brown.
Then roll out the pastry to line the tart tin, prick the base with a fork, place it on the pre-heated baking sheet, and bake in the centre of oven for 15 minutes. Then halve the plum tomatoes length ways and lay them across the base of your pastry (reserving all the tomato juices), then spread the onions all over them. Lay your strips of red peppers over the onions.
Finally warm your tomato juices, and 3 tbspns of the juice from the red pepper jar in a pan until thickened, pour over the top, return the tart to the oven and bake for 30 minutes at gas mark 5 till the edges of the pastry is golden brown.
serve with 320g peas and 320g carrots (80g per person)