Trim roots and dark green tops from leeks, leaving 5 to 8 inches of white and light green parts. Cut the leeks in half lengthwise (or in quarters, if large). Rinse well, taking care to remove any grit, and pat dry. Nestle the leeks in a single layer, on their sides if necessary, in a 9-by-13-inch baking dish. Drizzle with oil and sprinkle with herbs de Provence, pepper and salt. Cover with foil and bake until very tender, 50 minutes to 1 hour. Uncover and continue baking until lightly browned, about 10 minutes more.