Combine soy sauce, water, 2 tablespoons brown sugar, cognac, 1 tablespoon Dijon and garlic. Place beef steak in a large dish or plastic bag. Add marinade, turning to coat, cover dish or tie bag securely and marinate in refrigerator for 1 to 2 hours, turning occasionally.
Meanwhile, soak 4 (6") bamboo skewers in water 10 minutes. Combine butter and remaining brown sugar and Dijon mustard. Reserve. Skewer onion halves in 2 places through all layers, placing 2 halves on skewers. Repeat 4 times. Pour off marinade; discard. Grill steak and onions on grid over medium coals, turning once. Brush with reserved mustard mixture occasionally. Grill 14 to 16 minutes. Carve steak at an angle into thin slices. Serve steak with grilled onions. Garnish with tomatoes and parsley sprigs. 8 servings.
HINT: In general a steak should be at least 3" thick. Any steak that is more than 3" a thermometer should be used. 120 to 130 degrees will be very rare; 130 to 140 degrees rare; 140 to 160 degrees medium; 160 to 170 degrees well done. A 1 1/2" steak requires 14 to 16 minutes for a medium rare. Steak should be placed 4" above white hot charcoal.
