On rack in broiling pan, broil steak until rare, 3 to 4 minutes on each side.
While steak is cooking, in 10" non-stick skillet heat margarine over high heat until bubbly and hot; add onions and garlic and sauté, stirring occasionally, until onions are tender-crisp, 2 to 3 minutes.
Stir in broth, vinegar, and mustard and continue cooking until liquid is reduced by half, 3 to 4 minutes longer.
Transfer steak to 9x9x2" flameproof baking dish and top with onion mixture.
In small bowl combine 3 tablespoons of bread crumbs, parsley, and cheese, mixing well; sprinkle over onion mixture and broil until topping is lightly browned, 1 to 2 minutes.
Note: recipe from Weight Watchers.
There are 328 calories in 1 serving of Steak au Gratin.