Preheat a grill or grill pan to medium-high. Drizzle the bread with EVOO. Place on the grill or grill pan and cook, turning once, until toasty and charred in spots, about 3 minutes. Let cool slightly, then cut into cubes and place in a large bowl. Add 1/4 cup EVOO, the tomatoes, olives, onion, garlic, tarragon, lemon zest and lemon juice; season with salt and pepper. Let stand.
Drizzle the chicken with EVOO; season with salt and pepper. Add the chicken to the grill or grill pan and cook, flipping once and rotating a quarter-turn halfway through cooking on each side, until just firm, about 10 minutes. Serve the chicken on top of the panzanella.