Cook the Chicken: Season the diced chicken with salt, pepper, and fajita seasoning. Heat a pan over medium-high heat and cook the chicken until golden brown and fully cooked. Remove from the pan and set aside.

Sauté the Veggies: In the same pan, add the onions and bell peppers. Cook until softened, then stir in the garlic and cook for about 1 minute until fragrant.

Cook the Pasta: Add the uncooked pasta to the pan and mix well. Pour in the water (or broth) and chicken bouillon. Bring to a boil, then reduce the heat and add the milk. Let it simmer until the pasta is tender and the liquid has thickened into a creamy sauce.

Finish It Off: Stir in the Dijon mustard (if using) and shredded cheese, mixing until smooth and melted. Season to taste.

Combine & Serve: Add the cooked chicken back into the pan, mix everything together, and garnish as desired. Serve hot and enjoy!