How to Make It
Light a grill. In a small bowl, combine the onion with 3 tablespoons of olive oil, the vinegar, garlic, jalapeño and basil. Season the dressing with salt and pepper.
Thread six 12-inch metal skewers with the tomatoes, alternating with the mozzarella after every 2 tomatoes. Brush the chicken with olive oil and season with salt and pepper.
Grill the chicken over a hot fire until just cooked through, about 4 minutes. Transfer to a platter. Grill the skewers over a hot fire, turning once, until the cheese is lightly charred and just starting to melt, 1 minute. Slide the tomatoes and cheese from the skewers onto the platter. Drizzle 2 tablespoons of the dressing over the chicken and pass the remaining dressing at the table.