Preheat oven to 150 degrees C. Lightly grease a medium baking tray.
Melt 65g butter in a medium frypan over medium heat. Stir in onion, mushrooms and garlic. Cook until tender, about 10 minutes. Stir in 2 tablespoons flour, chicken stock, celery salt, white pepper and white wine. Reduce heat to low and cook, stirring frequently, until thickened and well blended, about 10 minutes. Mix 1/2 cup cheese into the thickened sauce mixture and stir until melted.
In a shallow medium bowl, mix 1 cup flour with salt and pepper. Coast chicken breast halves in the flour mixture. Melt remaining 65g butter in a large frypan over medium high heat. Cook chicken until lightly browned on all sides. Arrange breast halves in the prepared baking dish and cover with the sauce.
Top chicken breast halves with remaining cheese. Bake in the preheated oven 25 minutes, or until chicken is no longer pink and juices run clear.