Bring a large pot of salted water to a boil and cook the fettuccine until al dente according to package directions.
While the pasta cooks, heat the olive oil and butter in a large skillet over medium heat.
Add the sliced onion to the skillet and sauté until translucent, about 3–5 minutes.
Add the zucchini matchsticks to the skillet and continue to cook until slightly tender, about 3 minutes; then remove the skillet from the heat.
Drain the pasta, reserving 1/2 cup of the cooking water.
Toss the drained pasta into the skillet with the onions and zucchini and add some reserved cooking water as needed to keep the pasta moist and lightly sauced.
Place a large sheet of aluminum foil on a baking sheet, transfer the pasta to the center, cover with another sheet of foil, and seal the edges tightly.
Bake in a hot oven for 15 to 20 minutes until heated through.
Carefully open the foil (steam will be hot) and transfer the pasta to a large serving plate.
