• In a small bowl, combine the brown sugar, soy sauce, garlic powder, mustard and ginger. Place beef in a large resealable plastic bag; add 1/2 cup marinade. Seal bag and toss to coat. Place zucchini, yellow squash, red pepper and onion if desired in another resealable plastic bag; add remaining marinade and toss to coat. Refrigerate beef and vegetables for at least 4 hours, turning occasionally.
• Drain and discard marinade. On four metal or soaked wooden skewers, alternately thread beef and vegetables. Grill, covered, over medium-hot heat or broil 4-6 in. from the heat for 8-10 minutes or until meat reaches desired doneness, turning occasionally.
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