Christina's Beef and Tomato Kebabs

from Martha Stewart

• 1 1/2 pounds sirloin steak (about 1 1/4 inches thick or really any roast cut to that thickness) 
• 24 grape tomatoes 
• 2 tablespoons olive oil, plus more for grill 
• 2 tablespoons grainy mustard 
• 1 tablespoon red-wine vinegar 
• 1 tablespoon dried thyme leaves 
• Coarse salt and ground pepper 

1. Slice beef into cubes. Assemble 4 long skewers, alternating 5 beef cubes (threaded perpendicular to the cut) with 6 grape tomatoes on each. Arrange skewers in a nonmetallic dish. 
2. In a small bowl, whisk together oil, mustard, vinegar, and thyme. Pour marinade over skewers; turn to coat. Cover and let stand at least 2 hours in the refrigerator, turning periodically. 
3. Heat grill to high and lightly oil grates. 
4. Season skewers with salt and pepper and place them on grill. Cover grill and cook, turning occasionally, until grill marks are visible and meat is cooked to desired doneness, 1 to 2 minutes per side for medium rare.