Preheat the oven to 400 degrees . Arrange the baguette cubes on a rimmed baking sheet. Bake until toasted, about 10 minutes.
Meanwhile, heat a large grill pan over medium. Brush the chicken with 1 tsp. oil; season with salt and pepper. Grill the chicken until charred in spots and cooked through, about 5 minutes per side. Let the chicken rest 5 minutes. Cut into 3/4-inch squares.
In a large bowl, toss the tomatoes, cucumber, onion, feta, basil, vinegar, capers, remaining 2 tbsp. plus 2 tsp. oil, the chicken and the baguette cubes; season.