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 2 tbsp of olive oil in a large (12-inch) saute pan and add the onions. cook for 10 minutes on medium-low heat, until they start to brown. add half the zucchini, 1 tsp salt, and 1/4 tsp pepper to the pan and 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 repeat with the rest of the zucchini. serve immediately. note: if you cook too much zucchini in 1 pan, you end up steaming rather than sauteeing it.