For marinade, in a small saucepan combine onion, beer, Worcestershire sauce, brown sugar, garlic, bay leaf, and the 1/4 teaspoon pepper. Bring to boiling; reduce heat. Simmer, uncovered, for 4 to 5 minutes or until sugar is dissolved and onion and garlic are beginning to soften. Cool to room temperature. Diagonally score both sides of the steak at 1-inch intervals, making a diamond pattern. Place the steak in a plastic bag set in a shallow dish. Pour marinade over steak; close bag. Marinate in refrigerator for at least 4 hours or up to 24 hours, turning occasionally.
Drain steak, reserving marinade. Remove bay leaf. Season steak with salt. Grill on rack of an uncovered grill directly over medium heat to desired doneness, turning once. (Allow 12 to 14 minutes for medium doneness.) Meanwhile, for sauce, in a small saucepan combine reserved marinade and cornstarch. Cook and stir over medium heat until the sauce is thickened and bubbly. Cook and stir 1 minute more. To serve, cut steak across the grain into 1/8- to 1/4-inch-thick slices; spoon sauce over steak. If desired, sprinkle with parsley and additional pepper. Makes 4 servings.