01 - Bring a large pot of salted water to a rolling boil. Add the pasta and cook according to package directions until al dente. Drain through a colander and set aside.
02 - While the pasta cooks, melt the butter in a large skillet or Dutch oven over medium heat. Add the diced onion and sauté until softened and translucent, about 2 to 3 minutes. Add the minced garlic and cook for an additional 30 seconds until fragrant.
03 - Sprinkle the flour over the onion and garlic mixture. Stir continuously for about 1 minute to cook out the raw flour taste and form a smooth roux.
04 - Gradually whisk in the chicken broth followed by the milk, stirring constantly to prevent lumps. Bring the mixture to a gentle simmer and allow it to thicken for 2 to 3 minutes, stirring occasionally.
05 - Stir in the dried thyme, parsley, salt, and pepper. Add the frozen peas and carrots, corn, and cooked chicken. Continue cooking for 3 to 4 minutes until the chicken is heated through and the vegetables are tender.
06 - Add the drained pasta to the skillet and toss well to coat every strand evenly in the sauce. Taste and adjust salt and pepper as needed.
07 - If using cheddar cheese, stir it in now and mix until fully melted and creamy. Serve immediately while hot, garnished with fresh parsley if desired.