Preheat the oven to 450°F.
Turn the dough out onto a floured work surface. With your hands, flatten and stretch the piece of bread dough into a circle about 8 or 9 inches across. Use your fingertips and the heels of your hands to push down and outwards. Don’t worry: You can’t hurt this dough. If you have to struggle because the dough is springy and won’t hold its shape, use a rolling pin and that will flatten it out more quickly.
Pour 1 tablespoon olive oil in the middle of the circle. With fingers, spread it all over the dough.
Grease a baking sheet with vegetable shortening. Put the baking sheet next to your pizza dough. Loosen the dough with a spatula if it is sticking to the surface a little, and then, with your hands, carefully lift the dough and set it on the baking sheet. If it gets out of shape, pat it back into a circle with your hands. Turn the edges up a bit around the rim to make it neat, so it looks like a pizza.
Put the baking sheet in the oven and set the timer for 10 minutes. After 10 minutes, check the edges of the pizza. If they are still pale, bake another 3 to 4 minutes. If they are golden, the pizza is done. It usually takes about 12 to 15 minutes to bake completely.
While the pizza is baking, chop half an onion, grate enough cheese for ½ cup, and cut the tomato into 6 thick slices.
Remove the baking sheet from the oven with pot holders and set it on the cool stove top. If the dough has puffed up just give it a quick smack (it’s hot!) with your hand to flatten it before you put your toppings on. Put the tomato slices on the pizza, then sprinkle the onions and cheese evenly all over the top. Using pot holders, put the pizza back in the oven to bake for about 2 more minutes, just until the cheese melts. Remove the baking sheet from the oven with potholders, cut the pizza in slices or in half, and eat it right away.