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

While the pasta is cooking, dice the onion, mince the garlic, and cut the chicken breast into bite-sized pieces.

In a large skillet over medium heat, add olive oil. Once hot, add the diced onion and minced garlic. Sauté until the onion is translucent and fragrant, about 3-4 minutes.

Add the chicken pieces to the skillet. Season with salt and black pepper. Cook until the chicken is browned on all sides and cooked through, about 6-7 minutes.

Lower the heat slightly and pour the milk into the skillet with the chicken and onions. Stir to combine.

Gradually add the grated cheddar cheese to the skillet, stirring constantly to create a creamy sauce. Let it simmer for 2-3 minutes until the cheese is completely melted and the sauce thickens.

Add the cooked pasta to the skillet and toss to coat the pasta with the creamy sauce. Adjust seasoning with additional salt and pepper if needed.

Serve the creamy chicken pasta hot, garnished with a little extra grated cheese or fresh herbs if desired.