Prep all the ingredients: thinly slice the onion, mince the garlic, thinly slice the bell peppers, slice the zucchini in half length-wise and then slice into half-moons.
In a medium bowl, season the sliced chicken breast with 1 tablespoon fajita seasoning and salt (optional - check whether your fajita seasoning already contains salt).
Cook the pasta according to the instructions on the box. I recommend cooking until al dente. Drain and rinse the pasta and set aside. Time-saving tip: proceed with the next steps while the pasta is cooking.
In a large skillet, heat 1 tablespoon olive oil and add the seasoned sliced chicken. Cook over high heat for 5-7 minutes, or until the chicken is fully cooked. Set aside on a plate.
Add 1 tablespoon of olive oil to the same skillet and add the sliced onion and minced garlic. Saute for 2 minutes. Add the sliced bell peppers, zucchini, and the rest of the fajita seasoning. Season with salt, if desired. Stir fry over medium heat for about 5 minutes.
Add heavy cream, grated Parmesan cheese, and baby spinach to the skillet. Mix well. Add the cooked pasta and chicken, and continue stirring over medium heat until all the spinach is wilted (about 1-2 minutes). Remove from heat and stir in cilantro leaves.