Split the baguette in half lengthwise. Brush the cut sides with 2 tablespoons of the olive oil.

Heat outdoor grill to 375 degrees. When hot, place the bread slices, cut side down, on the grill to slightly char the bread. Alternately, the bread can be placed under a broiler to lightly toast.

Toss the remaining one tablespoon olive oil and balsamic vinegar with the cherry tomatoes. Place the tomatoes, with the oil and vinegar, in a 10-inch cast iron pan. Carefully place the pan in the center of the hot grill.

Cook the tomatoes for 3 to 4 minutes, stirring lightly a few times. The tomatoes should start to blister but not burst open. Use a hot pad to remove the pan from the grill. The tomato mixture can also be prepared inside on a medium-high burner.

Transfer the tomato mixture into a mixing bowl. Add the arugula or baby kale and toss lightly. Spread the pesto on the top piece of the toasted bread. Place the fresh mozzarella on the bottom piece of the bread. Top with the tomato/arugula mixture. Scatter the fresh basil and thinly sliced red onion over the tomato mixture. Place top of bread on the sandwich. If desired, the sandwich can be wrapped in foil and placed back on the grill or 375-degree oven for 2 to 3 minutes.