Into a baking dish, add some oil or olive oil just to grease the bottom. Then add the slices of bread, covering the whole bottom of the baking dish. Next, spread the tomato sauce over the slices, to moist them.
Add the mozzarella cheese, the sausage cut into round slices, the onion cut into round slices, the tomato chopped into round slices and top it all with some oregano to taste.
Bake in a preheated oven at 200°C (392°F) for 15 to 20 minutes.
Serve.