Preheat oven to 450°F. Heat grill to medium; oil grates. Grease a 2-quart baking dish with cooking spray.
Rub chicken all over with 2 Tbsp. oil, then season with salt and pepper. Grill, turning once, until cooked through, 12 to 15 minutes. Transfer to baking dish.
Brush bread with remaining 1 Tbsp. oil and grill, turning once, until golden, about 5 minutes; cut ciabatta into rough cubes. Scatter croutons around chicken. Spoon marinara sauce over chicken and top with cheese. Bake until cheese is melted and bubbling, about 10 minutes. Garnish with chopped basil leaves, if desired. Serve immediately.