Preheat the oven to 350ºF.
Put the fennel and onions in a roasting pan. Add the olive oil, salt, and pepper and toss to coat evenly. Roast until soft and lightly browned, about 1 1/2 hours. Let cool.
Combine the fennel mixture with the beans and toss to mix. Serve at room temperature.
Yield: about 6 servings