Stir the dry ingredients together for the dough. Add warm water and oil and stir until well combined. It should be a soft but manageable dough. Turn out onto a well-floured surface and knead well until the dough feels springy. Cover with the bowl and let it rest for 15 minutes to half an hour.
While the dough is resting grease a 15″x 10″ cookie pan and prepare topping ingredients. Turn on the oven to 400°F (204°C). Roll out the dough to a 16″x 11″ rectangle. Fit into the prepared pan. Dent surface of dough firmly with your fingers. Drizzle oil over top and brush to coat, allowing oil to puddle in the dents. Sprinkle with the remaining ingredients, laying tomato and onion slices on last, if using. Bake for 15 – 20 minutes or until nicely golden. Let cool for about 15 minutes before cutting into rectangles. A pizza cutter works really well for this. Cut in half for sandwiches with a serrated knife. Best served warm.
*Sometimes I carmelize the onions for the topping. That adds a nice touch. Simply fry the onions slowly in a pan with butter until they are deep golden in colour before adding on top.