Preheat oven to 350 degrees. Prepare salad dressing according to directions on package. Put in the fridge for a few minutes to set up. Saute onion until almost tender. Set aside.
Clean chicken breasts and sprinkle them with salt & pepper. Place aluminum foil in bottom of 13x9 baking dish, place chicken breasts in pan, and cover with more foil. Bake for 40 minutes. Remove from oven, let cool, and then shred the chicken with a fork. Mix shredded chicken with hot sauce and line bottom of another 13x9 pan. Spread chicken/hot sauce evenly in the bottom of pan.
Measure out 16 oz. of the Hidden Vallley dressing and pour into a a large bowl. Mix half of the bleu cheese (if it's an 8 oz. container). Try to mix it up where the cheese crumbles are tiny and the flavor well blended.
Combine the dressing/bleu cheese mix and cream cheese in a saucepan and heat until smooth enough to pour. Pour over chicken/hot sauce mixture. Sprinkle 2 cups of cheddar/Montery cheese on top, dot with a little butter and bake for about 30-40 minutes. Do not let the cheese on top burn.