Preheat the oven to 425 degrees F.
Take the giblets out of the chicken and wash it inside and out. Remove any excess fat and leftover pinfeathers. Toss the onion with a little olive oil in a small roasting pan. Place the chicken on top and sprinkle the inside of the cavity with salt and pepper. Place the lemons inside the chicken. Pat the outside of the chicken dry with paper towels, brush it with the melted butter, and sprinkle with salt and pepper. Tie the legs together with kitchen string and tuck the wing tips under the body of the chicken.
Roast for 1 1/4 to 1 1/2 hours, or until the juices run clear when you cut between the leg and the thigh. Cover with foil and allow to sit at room temperature for 15 minutes. (The onions may burn, but the flavor is good.)
Meanwhile, heat a large saute pan with 2 tablespoons of olive oil until very hot. Lower the heat to medium-low and saute the bread cubes, tossing frequently, until nicely browned, 8 to
10 minutes. Add more olive oil, as needed, and sprinkle with 1/2 teaspoon salt and 1/4 teaspoon pepper. Place the croutons on a serving platter. Slice the chicken and place it, plus all the pan juices, over the croutons. Sprinkle with salt and serve warm.