Pour ½ cup milk over the bread and leave to soak for a few minutes, add 2 tablespoons olive oil, the eggs, ½ cup grated cheese, salt, and pepper. Mash and stir with a fork until batter-like in consistency, add the sliced zucchini and the drained rice, mix well, and pour out into an oiled gratin dish. Press evenly all over, smoothing the surface with the tines of the fork, pour over the remaining milk (the contents should be bathing, the milk barely covering the surface), sprinkle evenly with the remaining cheese, dribble crisscross threads of olive oil over the entire surface, and bake in a 375° to 400° oven for 1 hour or until the surface is golden and the mass dry and firm to the touch.