Bring a large pot of water to a boil over high heat. Salt the water generously, and let it return to a boil. Add the pasta, and cook accordingly to package directions, stirring occasionally. Before you drain the pasta make sure to remove a cup of the cooking water.
While the pasta is cooking, heat the olive oil in a large skillet. Add the onions and sauté for 3 to 4 minutes until they start to soften and caramelize. Add the zucchini and sauté until it starts to become tender, about 3 minutes. Add the tomatoes and saute until they start to soften slightly release their juices, about 2 minutes.
Drain the pasta, reserving the cup of cooking water, and return the pasta to the pot with ½ cup of the cooking water. Add the zucchini tomato mixture to the pot, along with 3/4 of the feta and the lemon zest an juice and toss well. Season with salt and pepper to taste, and toss in the rest of the cooking water. Turn into a serving dish and sprinkle with the remaining feta (throw on a handful of chopped parsley if you have it handy). Serve hot or warm.