Remove the ends of the zucchini and, if they are large, cut in half lengthwise. Slice the zucchini diagonally in 1/2-inch slices. Heat olive oil in a large (12-inch) sautÃ© pan and add the onions. Cook for 10 minutes on medium-low heat, until they start to brown. Add the zucchini, salt and pepper to the pan to cook, tossing occasionally, for 10 to 15 minutes, until just cooked through. Sprinkle with Parmesan and cook for 30 seconds more. Remove to a serving platter and serve immediately.