In a kettle combine the beans with enough cold water to cover them by 2 inches, bring the water to a boil, and simmer the beans, uncovered, for 2 minutes. Remove the kettle from the heat and let the beans soak, covered, for 1 hour. Drain the beans in a colander, return them to the kettle, and stir in the onion and the 2 1/2 quarts water. Simmer the beans for 35 to 45 minutes, or until they are tender, drain them in the colander set over a large bowl, and transfer them to a 3-quart gratin dish or other shallow baking dish. Return the cooking liquid to the kettle and boil it for 5 to 10 minutes, or until it is reduced to about 1 1/2 cups. In a blender puree 1 cup of the beans with the reduced cooking liquid, the oil, the vinegar, and the salt and stir the puree with 1 1/2 cups of the watercress into the whole beans. The gratin may be prepared up to this point 2 days in advance and kept covered and chilled.

In a bowl toss together the bread crumbs, the Gruye, and the garlic, sprinkle the topping over the gratin, and bake the gratin in the middle of a preheated 425F. oven for 20 minutes, or until it is bubbly and golden. Divide the gratin among 6 to 8 plates and garnish each serving with some of the additional watercress.