1½ cups ketchup
3/4 cup Manischewitz wine (sweet kosher wine)
6 tbsp packed brown sugar
1 tsp freshly ground black pepper
4 garlic cloves, peeled and cut in half
1½ lbs sirloin steak, cut into 1½-inch cubes
1 red bell pepper, seeded and cut into 1½-inch pieces
1 yellow pepper, seeded and cut into 1½-inch pieces
1 green pepper, seeded and cut into 1½-inch pieces
½ lb button mushrooms, left whole, stems trimmed
In a medium bowl, whisk ketchup, wine, brown sugar and pepper. Pour 11/4 cups marinade into a large resealable plastic bag and add garlic halves, beef cubes, red peppers, yellow peppers, green peppers and mushrooms to marinade.
Seal bag, turn to coat and refrigerate at least 2 hours, up to 24 hours. Refrigerate remaining marinade to use for basting at the end of cooking.
Preheat barbecue to medium-high heat.
Remove meat and vegetables from plastic bag and discard marinade. Thread beef onto reusable metal skewers. Thread peppers and mushrooms on separate skewers from the beef to ensure even cooking.
Grill kebabs for 8-10 minutes, turning one-quarter rotation every 2-3 minutes or until the meat is cooked through and vegetables are tender-crisp. In the final 2 minutes of cooking time, baste all sides of beef and vegetable kebabs with reserved marinade.
Remove beef and vegetables from skewers and toss with extra marinade.