Preheat the oven to 425°F (218°C).
In a small roasting pan, toss the onion with a little olive oil.
Pat the chicken dry and place it on the onion. Sprinkle the inside of the cavity with salt and pepper and cram the lemons into the cavity. Brush the chicken skin with the melted butter and sprinkle with generous amounts of salt and pepper. Tuck the wing tips under the body of the chicken and, if desired, tie the legs together with kitchen string.
Roast the chicken for 1 1/4 to 1 1/2 hours, until the juices run clear when you make a slit between the leg and the thigh with the tip of a sharp knife.
Meanwhile, in a large skillet over medium-high, heat the 2 tablespoons oil until very hot. Reduce the heat to medium-low, add the bread, and let it toast, tossing frequently and adding more oil if needed, until golden brown, 8 to 10 minutes. Remove from the heat and sprinkle the bread with the 1/2 teaspoon salt and 1/4 teaspoon pepper.
Loosely cover the chicken with foil and let it rest at room temperature for 15 minutes. Grab a platter and place the croutons on it. Carve the chicken and place the pieces on the croutons. Spoon the pan juices over the chicken. Taste and, if desired, sprinkle with salt.