1. Rinse chicken; pat dry. Cut into thin, bite-size strips. In a large skillet, cook and stir chicken in hot butter for 2-3 minutes or until tender but no longer pink.
2. Stir in flour, seasoned salt, and mustard. Add milk all at once. Cook and stir over medium heat until thickened and bubbly. Cook and stir 1 minute more. Stir in cheese until melted, but do not boil.
3. For each serving, spoon chicken mixture over toast. Serve with tomato slices and sprinkle with green onions.