Make several days before, if desired, or on day:
1. In double boiler, over hot, not boiling, water, melt butterscotch pieces with 1 tablespoon shortening. Remove from heat; stir in chopped walnuts. Spread in well-greased 8-by-8-by-2-inch pan.
2. Arrange marshmallows evenly over butterscotch layer, gently pressing into surface.
3. In same double boiler, over hot, not boiling, water, melt chocolate pieces with 1 tablespoon shortening. Spread chocolate mixture evenly over marshmallow layer; lay walnut halves in soft chocolate.
4. Cool, then refrigerate until ready to serve. Cut into 48 bars.