Heat a large skillet with the olive oil over medium-high heat. Add the onions and sauté for 3-4 minutes until they begin to soften and slightly brown.
Add the garlic and cook for another 30-60 seconds until fragrant.
Add the milk, buffalo sauce, and black pepper to the skillet and continue cooking over medium-high heat, stirring often, to reduce the mixture to a thick sauce. This should take around 4-6 minutes. Once the sauce has thickened, remove from the heat.
Drain the cooked pasta and add to the sauce, tossing everything together, before adding the chicken. Fold everything together until the chicken is fully coated in the sauce.
Top with blue cheese crumbles and fresh green onion, if desired.