In a heavy kettle combine the endives, cut sides down, in two layers, the lemon juice, the butter, the salt, the sugar, and the broth, cover the mixture with a buttered round of wax paper and the lid, and bring the liquid to a boil. Simmer the mixture, covered, for 20 to 30 minutes, or until the endives are very tender, and transfer them with a slotted spoon, cut sides down, to a buttered gratin dish just large enough to hold them in one layer. In a small bowl stir together the Gruyere and the bread crumbs, sprinkle the mixture evenly over the endives, and broil the gratin under a preheated broiler about 4 inches from the heat for 3 to 4 minutes, or until the topping is golden and the cheese is melted. Serves 8