01 - Bring a large pot of salted water to a boil. Cook the pasta 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, about 2 to 3 minutes. Add the minced garlic and cook for 30 seconds more until fragrant.
03 - Sprinkle the flour over the onion and garlic, stirring constantly for about 1 minute to form a smooth roux and cook out the raw flour taste.
04 - Gradually whisk in the chicken broth followed by the milk, stirring until completely smooth. Bring the mixture to a gentle simmer and let it 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 everything is heated through and the vegetables are tender.
06 - Add the drained pasta to the skillet and toss well to coat every piece in the creamy sauce. Taste and adjust the seasoning as needed.
07 - If using, stir in the shredded cheddar cheese until fully melted and the sauce is creamy. Serve hot, garnished with extra parsley if desired.