The night before cooking, place the beans in a large bowl and cover them with water by 1 inch. Cover the bowl with plastic wrap and refrigerate overnight.
The next day, preheat the oven to 300 degrees. Drain the beans, rinse well, and drain again. In a large ovenproof pot such as Le Creuset, heat the olive oil over medium to medium-low heat, add the bacon, and cook for 4 to 5 minutes, until the bacon starts to brown. Add the onion, fennel, and carrots and cook for 7 minutes, stirring occasionally, until the vegetables begin to soften but aren't browned. Add the garlic and saute for 1 to 2 more minutes.
Add the flageolets to the pot and stir in the broth, bay leaves, and rosemary. (Don't be tempted to add salt! The beans will become tough.) Add 2 cups of water, which should just cover the beans, and bring to a simmer on top of the stove. Cover the pot tightly and bake in the oven for 45 minutes. Remove the lid, stir in 1 tablespoon salt and 1 teaspoon pepper, and return the pot to the oven without the lid. Raise the temperature to 350 degrees and bake for 30 to 45 more minutes. The beans will be very tender and there will be just a little liquid in the bottom of the pan. (If the beans are dry, add a little more water.) Discard the bay leaves and rosemary. Taste for seasonings and serve hot.
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