Preheat oven to 375. Dice onion and garlic and spread on chicken breast. Wrap chicken in tinfoil, put on oven safe tray, place on center rack and cook for 16 minutes. Turn chicken over and bake for another 16 minutes. Remove from oven and let stand for 5 minutes. Unwrap and allow to cool enough so that you can shred it.
On medium heat, melt butter in small sauce pan. Once butter is completely melted, add hot sauce and apple cider vinegar to the sauce pan and whisk until fully combined. If you like hotter buffalo dip, add cayenne pepper powder. Whisk periodically until the sauce is bubbling, and remove from heat. You can do this while the chicken is cooking.
To a medium-sized bowl, add softened cream cheese and ranch dressing. Combine fully before adding the buffalo sauce. Once sauce is added, stir until no longer lumpy. You can also do this while chicken is cooking.
Decrease oven temp to 350. Add shredded chicken, garlic and onions to mixture. Move mixture to an 11 x 7 baking dish. Evenly distribute cheese on top. Bake uncovered for 20-25 minutes, or until cheese is bubbling all over. You can always broil at the end if need be. Remove from oven when done.
Chop and add scallions (optional).
Serve with tortilla chips or whatever you might like.