Cut the chicken into small cubes and sprinkle with olive oil. Season with salt, black pepper, fajita seasoning, chili powder, smoked paprika, cumin, and garlic powder. Mix well to coat the chicken evenly.
Heat a pan over medium heat and add olive oil. Cook the chicken for 5-7 minutes, tossing until cooked through. Remove the chicken from the pan and set aside.
Add butter to the same pan. Add the finely chopped onion and sauté for 4 minutes until translucent. Add the diced bell pepper and minced garlic, stirring well and cooking for another 1-2 minutes.
Add the dry pasta to the pan, followed by chicken broth and milk. Reduce the heat to low and let the pasta simmer until almost cooked and the sauce has thickened slightly. Taste and adjust seasoning with salt, cayenne, or fajita spices.
Once the pasta is al dente, remove the pan from heat. Stir in the grated cheddar and mozzarella cheese until melted and smooth. Return the cooked chicken to the pan and mix well.
Garnish with finely chopped cilantro or parsley before serving. Enjoy your delicious Fajita Mac & Cheese!
