Preheat the oven to 350 degrees.
Between waxed paper, pound chicken to ¼ inch thickness.
Sprinkle with salt and pepper and dust with flour. Melt ¼ c. butter and quickly sauté breasts, a few at a time until golden.
Remove to plate and set aside.
Melt remaining butter in same skillet and sauté onion, garlic and mushrooms slowly until cooked, not brown.
Stir in flour, celery salt, white pepper, chicken stock and juice.
Cook over low heat until thickened, about 4-5 mins.
Stir in mashed avocado and ½ c. cheese and season to taste.
Arrange breasts in 9x13 pan and spoon avocado mix on each and sprinkle with remaining cheese over top of chicken breasts.
Bake 20-30 mins at 350 until chicken is cooked and cheese is melted.