Bring a large pot of salted water to a boil and cook the pasta according to package instructions until al dente. Reserve 1/2 cup of the pasta cooking water, then drain the pasta and set aside.
Toss the chicken pieces with the fajita seasoning until evenly coated.
Heat the olive oil in a large skillet or heavy-duty pan over medium-high heat. Add the seasoned chicken and cook, stirring occasionally, for 2–4 minutes until browned and cooked through.
Add the sliced bell pepper, sliced onion, and minced garlic to the pan. Sauté for 2–3 minutes, until the onion is tender and the peppers have softened.
Pour in the heavy cream and the reserved 1/2 cup pasta water, then bring the mixture to a gentle simmer.
Add the drained pasta and shredded cheddar to the pan. Stir well and simmer for about 5 minutes, stirring occasionally, until the cheese is melted and the sauce coats the pasta. Add a splash more pasta water if needed to loosen the sauce.
Serve immediately, optionally topped with fresh minced cilantro.