Preheat oven to 200 degrees C. Roll out pastry, and use to line a 23cm flan tin. Line pastry with a sheet of greaseproof paperand alayer of baking beans. Bake blind for 10 minutes, remove paper and beans.
To make the filling, place the sun-dried tomatoes in a bowl. Add 1 cup boiling water and leave to soften, about 10 minutes.
In a heavy-based frying pan, heat oil over a medium heat. Add onions, cover and saute for 3 minutes, shaking pan frequently. Turn the onions over, cover and saute for further 3 minutes, shaking pan. Reduce heat to low, and saute for 6 minutes, or until soft.
Drain the tomatoes and chop them finely. Add tomaotes to onions with salt and pepper. Mic well. Spread the mixture in the pastry case. Cover with tomato slices. Place olives around edge and sprinkle with oregano.
Spray the tomatoes with olive oil cooking spray. Bake for 30 minutes, until tart is cooked. Serve.