1 pound dried white beans such as Great Northern or navy
picked over
4 cups chopped onion
2 1/2 quarts water
3 tablespoons olive oil
2 teaspoons white-wine vinegar
1 teaspoon salt
1 1/2 cups packed tender watercress sprigs plus additional sprigs for garnish if desired
1 cup fine day-old bread crumbs
1 cup grated Gruyere
1 garlic clove
minced
Instructions
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.