Put the flour, butter, vegetable fat and salt into a food processor and pulse until the mixture resembles breadcrumbs.
Add the egg and the water and pulse until the mixture starts to come together, add a little more water if it is too dry.
Press the mixture into a ball and wrap in plastic wrap. Chill in the fridge for 20 minutes minimum.
Pre -heat the oven to 200C
Roll out the pastry thinly in a circle and line the tart tin. Trim the pastry around the edge of th tin but leave enough to allow for a little shrinkage.
Place a piece of baking parchment onto the pastry and top with baking beans or dried peas, beans or rice.
Bake blind for 15-20 minutes, then remove the paper and beans and bake for another 5 minutes.
Remove from the oven and leave to cool.
For the Tomato & Bacon filling
Reduce the heat of the oven to 170C
Fry the bacon in a pan on medium-high heat until the fat runs off and they are golden brown and crisp. Remove from the heat and lift them out with a slotted spoon and place on a plate with kitchen paper to drain
Put the bacon pieces in the base of the blind-baked pastry case. Cut the tomatoes in half and arrange over the bacon to fill the pastry case in a single layer.
Beat the eggs and cream together with a fork or a whisk, and season with salt.
Cut the chives with scissors into small pieces, no more than 1/2 cm long. Add these to the custard mixture.
Finally, pour over the egg, cream, and chives mixture and bake in the oven for 35-40 minutes until golden brown and set.
Remove from the oven and leave for 10 minutes before removing from the tin.
Without a Food Processor
If you do not have a food processor then put the flour, butter, and vegetable fat into a bow. Rub the fats into the flour with your fingers until the mixture is like breadcrumbs, then add the rest of the dry ingredients and combine. Add the egg and water and mix with a fork or your fingers until you have a ball of dough.
Cool the finished tart, then wrap tightly in cling film and foil. Freeze for up to 3 months. Defrost at room temperature and warm in a medium oven for 20 minutes.