We have tart about once every two weeks, usually leek, pancetta/bacon and pea.
Roll out pastry (if it's already rolled, keep it on its paper and transfer to baking tray), score an edge about 2cm round and then prick the centre with a fork. Cook for 10 minutes at about 180.
Meanwhile, saute leeks in a bit of oil/butter until soft. Remove and set aside then fry slices of pancetta until almost browned (they'll cook more in the oven).
In a bowl, mix a tub of cream cheese, a generous teaspoon of dijon mustard, one egg yolk, a generous grating of parmesan cheese and some chopped or dried herbs of your choice - I usually use thyme and oregano as that's what I have lying around but chives and dill would be nice I think. Salt and pepper.
Bash down the centre of the pastry and roughly spread the cream cheese mixture. Top with the leeks and pancetta and scatter defrosted frozen peas. Grate some more parmesan over the top.
Back in the oven for about 10 minutes or so.
Usually we have that with salad.
The pesto/tomato option someone mentioned above is great too. Variation is to do it with red pesto rather than green. Sprinkle the tomatoes with dried oregano for a little italian hit.
I've also used goats cheese and tomato.