Remove ends of zucchini and cut them in half lengthwise. Then slice them diagonally in 1/2" slices.
Heat olive oil in large sauté pan and add the onions. Cook for 10 minutes on medium low heat until they just start to brown. Add the zucchini, salt(I use Kosher salt) and pepper. Toss occasionally for 10-12 minutes or until just cooked through. Al dente, please no soggy zucchini. Sprinkle with grated Parmesan and cook for about 30 seconds more. Remove to serving platter and serve immediately.
Don't add too much zucchini to pan or it will steam instead of sauté. Cook in two batches, if necessary.