In a small bowl, mix together the dry mustard, dried thyme, dried oregano, salt, and black pepper. Set aside.
Slice the chicken into thin strips or 1-inch cubes. Season evenly with the spice mixture.
Warm the olive oil in a large skillet over medium-high heat. Add the chicken and cook until browned and fully cooked through. Remove the chicken to a plate and set aside.
Bring a large pot of salted water to a boil. Add the rotini and cook until al dente according to package directions. Drain and set aside.
In the same skillet used for the chicken, melt the butter. Once melted, stir in the flour to form a smooth paste. Add the chicken broth, garlic, onion, and Dijon mustard, stirring well. Cook for 3–5 minutes, or until the onions begin to soften.
Gradually pour in the milk, a little at a time, stirring after each addition so the sauce stays smooth. Simmer on low for 4–6 minutes, or until it thickens. Add the grated white cheddar and stir until melted.
Return the cooked chicken to the skillet, add the pasta, and toss until everything is fully coated in the creamy sauce.
Serve immediately. Garnish with fresh thyme, crushed red pepper flakes, or a sprinkle of smoked paprika if desired.
NutritionCalories: 659 kcal | Carbohydrates: 67 g | Protein: 39.6 g | Fat: 25.8 g | Saturated Fat: 15 g | Cholesterol: 120 mg | Sodium: 750 mg | Potassium: 750 mg | Fiber: 3 g | Sugar: 6 g | Vitamin A: 1200 IU | Vitamin C: 5 mg | Calcium: 350 mg | Iron: 3 mg