Combine lamb, onion, and French dressing in a shallow dish; cover and marinate at least 1 hour.
Combine gelatin and 1/4 cup water in a large mixing bowl; stir to soften gelatin. Set aside.
Combine pineapple juice, vinegar, 1/2 cup water, sugar, and salt in a small saucepan. Bring to a boil. Remove from heat; pour over gelatin. Stir until gelatin dissolves. Pour one-fourth of gelatin mixture into a lightly oiled 3 1/2-cup ring mold. Chill until partially set.
Arrange olive slices around ring over gelatin. Chill until set.
Pour one-half remaining gelatin mixture into mold. Chill until set.
Drain lamb; discard marinade. Spoon lamb over gelatin in mold.
Combine remaining gelatin mixture, peppermint extract, and food coloring. Spoon over lamb in mold. Chill until firm. Invert onto a lettuce-lined platter. Garnish center with mint leaves.