In a ziplock back place chicken breasts. In a bowl combine salt and sugar. Pour in buttermilk and stir to combine. Pour into bag and place in fridge 4 hours up to overnight.
When you are ready to cook drain the brine and remove the chicken breasts. Cut your ziplock bag on the edges except the bottom. This makes a really convenient way to flatten your chicken breasts with less mess.
Flatten the chicken breasts by half their original thickness. Then cut them in half to create 4 portions.
At this point let the chicken portions rest while you work on your mushrooms. In a medium pan heat oil and add sliced mushrooms. Cook until the mushrooms begin releasing juice. Add worcestershire sauce and italian seasoning and continue cooking until almost dry. Season with salt and pepper to taste.
Spray a grill pan with cooking spray. Add Chicken breast to pan. Cook on medium high heat after 2 minutes give the chicken breasts a quarter turn - this creates cross marks. Cook 2 minutes more on that side. Flip the chicken breasts over and repeat the process Turn off heat top with mushrooms and then cheese. Place lid on pan and allow cheese to melt.
Flip the chicken breasts over and repeat the process Turn off heat top with mushrooms and then cheese. Place in oven and allow cheese to melt.
Spread top of roll with horseradish sauce and the bottom of roll with spicy mushroom. Place chicken on bottom roll. Additional toppings as desired.