Beat chicken breast until even in thickness. In a bowl, combine seasonings for chicken. Season both sides of the chicken breasts.
In a large skillet, over med/high heat, heat 2 Tbsp oil. Sear chicken 2-3 minutes per side. Turn heat down to low and cook another 5-6 minutes, until the chicken is tender and fully cooked.
Remove the chicken from the skillet and cover to keep warm. In the same skillet, saute the mushrooms with chopped onions and minced garlic. Cook until mushrooms are golden, salt and pepper to taste.
Meanwhile, peel the potatoes slice into thin slices. Over medium/high heat, saute the potato for about 10 minutes or until potatoes are tender. Season the potatoes with salt and pepper.
In a bowl, whisk together the sour cream, egg, mayo, milk, salt and pepper.
Once the potatoes are tender, transfer them into a casserole dish. Spread them evenly along the bottom of the casserole dish. Top with chopped greens.
Slice the chicken breast into thin strands. Spread over the potatoes in the casserole dish.
Top chicken with the mushroom mixture. Place 4 Tbsp of butter into the dish.
Combine all the ingredients for the sauce and pour the sauce all over the casserole.
Add the cheese and cover dish with foil.
Baked at 400°F for 35 minutes, remove the foil and bake an additional 10 minutes.