Make a fairly thick porridge, stirring with a wooden spoon over a low flame for about 40 minutes. ( You could use a quick-cooking polenta if you wanted to significantly cut down on cooking time.)
Separately, sauté the onions in a trickle of oil in a pan.
Add the sausage and raise the heat to cook the mixture through.
Add tomatoes and allow to reduce.
Place the cooked cornmeal on a rectangular oven tray and leave to cool.
Cut the cooled cornmeal into rectangles which are not too thin.
Deep fry the cornmeal slices in oil. If you want to prevent the polenta from crumbling, let the cornmeal slices color nicely on each side before turning them. Also be sure to maintain a high oil temperature.
When they are crisp and golden, liberally spread the sausage mixture over them.