Makes one shallow 9 inch tart - double the filling if you want it deep-filled.
For the pastry:
For the filling:
Preheat the oven to 200°. To make the pastry, rub the fat into the flour until you get coarse breadcrumbs. Stir in the salt and add just enough cold water to form a dough - start with a couple of spoonfuls and only add more if you really need it to get the mixture to clump. Gently roll out on a floured surface until just large enough to fit your flan tin. Line the tin with the pastry, making sure there are no cracks or splits, line with greaseproof paper and weight down with baking beans, old pulses or similar. Bake blind for about 7 minutes, until the sides are sufficiently cooked to support themselves. Remove the beans and paper lining and continue cooking until the base is dried out too. Turn the oven down a tad, or cover the sides of the pastry case if the edges start to brown before the centre is done.
Meanwhile, make the filling. Cut any excess fat off the bacon and then chop into raggedy strips. Dry fry in a hot pan, making sure to evaporate off any liquid which seeps out. Then add the oil. Chop the mushrooms in half and then slice finely and add to the pan. Rinse out any mud then slice the leek into 3-5mm rounds and chuck them in the frying pan too. Cook over a medium heat for about 5 minutes, until all are cooked through. Whisk the eggs with the mustard and milk.
Once the pastry case has finished baking and the filling is cooked through, assemble the tart by simply spreading the bacon and vegetables mixture evenly over the base, pouring in the egg mix and grinding a little pepper over the top. Place it back in the oven and cook until the filling is set, which should take about 10 minutes. Eat hot, warm or cold, with salad and maybe some chutney on the side, view of the Thames optional.