Cook the pasta:
Bring a large pot of salted water to a boil. Add pasta and cook according to package directions until al dente. Drain and set aside.

Prepare and cook the chicken:
Pat the chicken breasts dry. In a small bowl, mix olive oil, dry mustard, thyme, oregano, salt, and pepper. Rub mixture over chicken. Heat a skillet over medium-high heat, add a little oil, and sear chicken for 5–6 minutes per side or until cooked through. Remove and let rest before slicing.

Start the sauce:
In a large saucepan, melt the butter over medium heat. Add diced onion and sauté until translucent, about 3–4 minutes. Stir in minced garlic and cook for 1 minute until fragrant.

Build the roux:
Sprinkle flour over the onion mixture and stir to combine. Cook for 1–2 minutes, stirring constantly, to eliminate raw flour taste.

Deglaze with wine:
Slowly pour in the white wine, stirring to loosen any bits from the pan. Let it simmer for 2 minutes.

Add milk and mustard:
Gradually whisk in milk. Add Dijon mustard and stir. Let the mixture simmer for 5–6 minutes until slightly thickened.

Melt in the cheese:
Lower the heat and add the grated white cheddar, stirring until melted and smooth.

Season the sauce:
Add crushed red pepper flakes and fresh thyme to taste. Adjust seasoning with salt and pepper if needed.

Assemble the pasta:
Add the cooked pasta and sliced chicken to the sauce. Stir until everything is well coated.

Serve:
Serve hot, garnished with extra thyme or shredded cheddar if desired.