Remove all fat from chicken and sprinkle with the seasoned salt. Place the chicken in a casserole dish and cover with foil. Bake the chicken in the oven for 1 hour.
After 30 minutes, begin chopping the spinach, removing the stems. Saute the mushrooms in two Tablespoons of butter until softened and set aside. Saute the spinach with the remaining butter, until wilted.
Mix the spinach with the mushrooms.
Remove chicken from pan and place on each plate while still very warm.
Top each piece of chicken with 1/2 of mushroom and spinach mixture plus 1 slice of cheese.