Soak beans overnight in cool water. Rinse and set aside.
Halve leeks lengthwise and cut crosswise into 1/2-inch pieces, then wash well and pat dry.
Sauté leeks, carrots, celery, and garlic in oil with herb sprigs, bay leaf, cloves, and 1/2 teaspoon each of salt and pepper in a large heavy pot over medium heat, stirring occasionally, until softened and golden, about 8 minutes. Stir in beans, then water, and simmer, partially covered, stirring occasionally, until carrots are tender but not falling apart, about 30 minutes.
Preheat oven to 350°F
Discard herb sprigs and bay leaf. Mash some of beans in pot with a potato masher or back of a spoon to thicken broth. Season with salt and pepper.
Divide mixture evenly between 4 large ramekins, sprinkle with 1/4 cup of bread crumbs and bake in the oven for 30 minutes.
Using a wooden spoon, gently press the thin crust that has formed on the cassoulet down under the surface.
Continue baking until the cassoulet is simmering and a second thin crust has formed (about 30 minutes).
Let stand for 10 minutes before serving.