Whisk together Marinade ingredients in a small bowl, place in a zip top plastic bag; add flank steak. Seal the bag and massage the marinade all over the steak. Refrigerate for at least 3 hours, preferabley overnight.
In a small bowl, whisk together sour cream , mayonnaise, horseradish and mustard.
Season with salt and pepper to taste.
Cover with plastic wrap; refrigerate while preparing steak.
In a small bowl mix roasted red peppers, garlic and basil leaves, set aside.
Heat broiler, if using, with rack 6 inches from heat.
Remove steak from marinade; shake off excess marinade and place on a roasting pan fitted with a rack, if broiling.
BBQ or broil 6 minutes on each side for medium rare. Let rest 5 minutes before slicing 1/4 inch thick against the grain.
Place watercress or cilantro in a bowl; toss with oil and the lemon juice. Season with salt and pepper.
Toast bread slices; place 1/4 watercress mixture on bottom slice of toast; place 1/4 of steak slices over watercress; drizzle horseradish sauce over steak; place 1/4 of roasted red pepper mixture over sauce; top with second slice of toast. Repeat for 4 sandwiches.