Cut the 2 boneless, skinless chicken breasts into cubes and season with salt and pepper to taste.

Heat 1 tablespoon of the unsalted butter in a Dutch oven or heavy‑bottomed pot over medium‑high heat. Add the cubed chicken and cook, stirring occasionally, 5–7 minutes until lightly browned and cooked through. Transfer the chicken to a plate and set aside.

Add the remaining 1 tablespoon unsalted butter to the same pot. Add the minced garlic and cook 1–2 minutes, stirring, until fragrant.

Pour in the 2 cups chicken stock and 1 cup milk, then stir in the 1 teaspoon Dijon mustard. Bring the mixture to a boil.

Once boiling, add the 2 cups dry elbow macaroni and stir to combine. Reduce heat to medium‑low and simmer, stirring regularly, 9–10 minutes until the pasta is al dente.

Remove the pot from the heat. Add 1 cup freshly grated cheddar cheese, ½ cup freshly grated low‑moisture mozzarella, and ½ cup freshly grated Parmesan. Stir until the cheeses have melted and the sauce is smooth. Season with additional salt and pepper to taste.

Return the cooked chicken (and any accumulated juices on the plate) to the pot, fold into the macaroni and cheese until evenly combined, and serve immediately.