Balsamic Beef Kabob Sandwiches

Ingredients:
- 1/4 cup balsamic vinegar
- 1/4 cup olive oil
- 2 garlic cloves, minced
- 1 teaspoon dried rosemary, crushed
- 1/2 teaspoon pepper, divided
- 1/4 teaspoon salt, divided
- 1-1/2 pounds beef top sirloin steak, cut into 1/4-inch-thick strips
- 2 medium onions
- 8 naan flatbreads
- 2 cups chopped heirloom tomatoes

Directions:
- Mix first four ingredients; stir in 1/4 teaspoon pepper and 1/8 teaspoon salt. Toss with beef; let stand 20 minutes.
- Cut each onion into eight wedges; thread onto metal or soaked wooden skewers. Thread beef strips, weaving back and forth, onto separate skewers.
- Grill onions, covered, over medium heat until tender, 5-7 minutes per side. Grill beef, covered, over medium heat until desired doneness, 3-4 minutes per side. Grill flatbreads until lightly browned, 1-2 minutes per side.
- Toss tomatoes with the remaining pepper and salt. Remove onion and beef from skewers; serve on flatbreads. Top with tomatoes.