Season chicken with seasoning salt and let set about 15 minutes. Meanwhile, melt butter in frying pan. Saute mushrooms and onions/shallots; add salt and pepper. Cook about 10 minutes. Remove onions/mushrooms from pan and set aside. Add chicken to pan and cook about 10 minutes on each side (it won't take as long for chicken tenders). When chicken is done, cover chicken with onions/mushrooms. Top with cheese and let it melt on top of chicken mixture. If you have some of the liquid left in pan, spoon over chicken or dip some toasted French bread in it.