Veggie-Loaded Creamy Chicken Fajita Pasta

yellow onion (thinly sliced)

garlic cloves (, minced)

bell peppers (, seeds removed and thinly sliced)

small zucchini ((about 1/2 lb))

pasta (, such as penne or ziti)

water and salt for cooking pasta

chicken breast ((2 small chicken breasts), thinly sliced)

fajita seasoning (, divided (or to taste))

Optional: salt (, to taste )

olive oil (, divided)

heavy cream

grated Parmesan cheese

baby spinach

cilantro leaves ((optional))