Preheat oven to 425 degrees. Cut chicken along backbone on 1 side with kitchen shears and open it like a book. Turn over and press to lay flat. Slather with 2 tablespoons butter; season all over with salt.
Spread remaining tablespoon butter on 1 side of bread slices. Place side by side, buttered side up, on a parchment-lined rimmed baking sheet and lay chicken on top. Roast until chicken is golden brown and a thermometer inserted into thickest part of breast (do not touch bone) reaches 160 degrees, 30 to 35 minutes.
Remove chicken from oven; let rest on baking sheet 10 minutes. Scatter with herbs; squeeze lemon juice over chicken. Cut chicken and bread into serving pieces; place on a platter. Pour pan juices over chicken and serve.