MIX mustard and mayonnaise; cover. Refrigerate until ready to use.
FOR each serving: Spread each of 2 bread slices with 2 Tbsp. of the mayonnaise mixture. Place 3 ham slices, 4 cheese slices, 1 tomato half and 1/2 cup of the arugula on 1 of the bread slices; top with second bread slice. Cut in half to serve.