COMBINE tuna, mayonnaise, 3/4 cup of the mustard (or 2 Tbsp. of the mustard for trial recipe), pickles, celery, hearts of palm, onions, parsley and pepper. Refrigerate until ready to use.
FOR each serving: Spread 1 toast slice with 1 Tbsp. of the remaining mustard. Top with 1/2 cup of the tuna mixture and 2 each tomato slices, bacon slices and lettuce leaves. Cover with second toast slice.