In a large roasting pan, arrange the endives cut-side down in a single layer, overlapping them slightly. Scatter the butter on top and season with pepper. Roast the endives for about 1 hour, until tender, turning them once or twice. Spoon off any liquid in the roasting pan and continue to roast the endives for 15 to 20 minutes longer, turning occasionally, until golden and lightly caramelized. Transfer to a platter; serve.