Marinade:
1/2 cup dark rum
2 tablespoons brown sugar
1 tablespoon coarsely ground black pepper
1/4 teaspoon salt
5 garlic cloves, crushed, or 2 1/2 teaspoons bottled minced garlic
1 (1 1/2-pound) flank steak

Flavored mayonnaise:
1/2 cup fat-free or light mayonnaise
2 teaspoons prepared horseradish

Remaining ingredients:
8 (1/2-inch-thick) slices red onion (about 2 onions)
16 (1-ounce) slices sourdough bread
2 cups thinly sliced romaine lettuce
16 (1/4-inch-thick) slices tomato (about 3 tomatoes)

To prepare marinade, combine first 5 ingredients in a large zip-top plastic bag. Trim fat from steak, and add steak to bag. Seal and marinate in refrigerator at least 2 hours, turning bag occasionally. Remove the steak from bag, reserving marinade. Pour the marinade into a microwave-safe dish. Microwave marinade at HIGH 1 minute or until mixture boils.
To prepare flavored mayonnaise, combine mayonnaise and horseradish, and set aside.
Prepare grill.
Place steak and red onion slices on a grill rack, and grill onions 4 minutes on each side, basting with the reserved marinade. Remove onions from grill. Turn the steak, and grill an additional 8 minutes or until steak is desired degree of doneness. Remove steak from grill. Place bread slices on grill rack, and grill 2 minutes on each side or until toasted.
Cut steak diagonally across grain into thin slices. Spread 1 tablespoon flavored mayonnaise on one side of each of 8 toasted bread slices. Divide steak, onion slices, lettuce, and tomato slices evenly among 8 bread slices. Top with remaining bread slices.
Note: Substitute pineapple juice for rum, if desired.
Yield 8 servings (serving size: 1 sandwich)

Nutritional Information
CALORIES 345(29% from fat); FAT 11g (sat 4.4g,mono 4.6g,poly 0.7g); IRON 3.8mg; CHOLESTEROL 45mg; CALCIUM 84mg; CARBOHYDRATE 39.2g; SODIUM 640mg; PROTEIN 22.6g; FIBER 2.6g
Elizabeth Taliaferro , Cooking Light, MAY 1999