From YavelWiki

Prep Time: 20 mins; Cook Time: 10 mins; Serves: 4

Ingredients

• 1/4 cup Worcestershire sauce
• 1/4 cup balsamic vinegar
• 4 tablespoons soy sauce
• 1 tablespoon dijon mustard
• 1 tablespoon minced garlic
• 1 tablespoon brown sugar
• 1 teaspoon black pepper
• 1 1/2 pounds beef sirloin tips (or similar grilling steak) cut into 1½ inch cubes
• 1 large red onion, sliced into 1½ inch pieces

Instructions

1. Whisk together Worcestershire sauce, vinegar, soy sauce, Dijon mustard, garlic, brown sugar, and pepper. Add to a bag along with beef. Seal and marinate for 20 minutes or up to 14 hours (the longer the better!)
2. When ready to prepare, grease and preheat the grill. Skewer the onions and beef and grill over medium heat 8-10 minutes, turning throughout, until just cooked through or desired doneness is achieved. Serve garnished with parsley or cilantro if desired and steak sauce for dipping.