In a large pot of boiling salted water cook the pasta until al dente.
Meanwhile in a large skillet, over medium heat, sauté the onion with the butter and olive oil. When translucent add the zucchini and continue cooking until slightly tender, 3 minutes. Turn off heat and set aside.
Drain the pasta, reserve ½ cup of cooking water. Add the pasta to the skillet with the onions and zucchini, add some of the reserved cooking water to insure the pasta is nicely moist
Place a large sheet of aluminum foil in a cookie sheet and add the pasta in the center of the aluminum foil. Cover with another sheet of aluminum foil and seal the edges well together.
Place in the hot oven and bake for 15 to 20 minutes.
Remove from the oven, open the foil – be careful quite hot - and place the content in a large pasta serving plate.