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 150°C. 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 sauté 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 175°C 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.