Spanish Potato Tortilla with Onions and Sausage

Peel the potatoes and onions and cut them into thin slices. Rinse the bell peppers, cut in half, remove seeds and ribs and cut into small cubes. Peel chorizo and cut into slices. Heat the oil in a skillet and add the vegetables. Add 1/2 cup water, bring to a simmer, cover and cook over medium heat for about 10 minutes.  Remove the cover and cook over high heat until the liquid has evaporated. Add the sausage.