Mix the flour, salt, sugar and yeast together in a bowl. Add the oil and measurement lukewarm water and mix with your hand until the mixture comes together into a dough and leaves the sides of the bowl clean. If too dry, add a little more lukewarm water.
Turn the dough out on to a lightly floured surface and knead for about 10 minutes until smooth and stretchy. Put in a clean bowl, cover with clingfilm and leave to rise in a warm place for about 1 hour until doubled in size.
Meanwhile, make the topping. Heat the oil in a large frying pan, add the onions, garlic, thyme and dried herbs and cook, covered, over a gentle heat, stirring occasionally, for about 30 minutes until the onions are meltingly soft.
Turn the dough out on to a lightly floured surface and knead for 1 minute. Roll out the dough and use to line a 33 x 23 cm (13 x 9 inch) Swiss roll tin. Spread the cooked onions over the top, scatter over the olives, tomatoes and capers and season with salt and pepper. Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 25 minutes until golden. Garnish with thyme leaves and serve warm or cold.