STEAK: In a 9 x 13-inch glass baking dish, whisk together soy sauce, brown sugar, vinegar, Worcestershire sauce, mustard, red pepper flakes, and pepper. Place steak in dish, turn steak over to coat the other side, then cover dish tightly with plastic wrap. Refrigerate or 2 hours, turning steak occasionally.
Clean and lightly oil grill grates. Heat grill to medium-high. Remove steak from marinade, allowing excess to drip back into the baking dish. Pour marinade into a saucepan and simmer over medium-high heat until reduced by half, about 10 minutes.
Grill steak covered for 4-5 minutes; flip steak over and brush with glaze. Grill until medium-rare, 4-5 minutes more. Allow meat to rest for 8 to 10 minutes before thinly slicing against-the-grain. Serve steak over salad.
SALAD: Arrange lettuce, tomatoes, red onion, bleu cheese crumbles, and pecans on a serving platter. Serve sliced steak over the top. Drizzle some glaze over the steak. Drizzle bleu cheese dressing over all OR simply pass the dressing at the table.