Place the beans in a large bowl, cover with cold water, and soak overnight.
Drain well.
Transfer the drained beans to a 6-quart Dutch oven and add enough fresh water to cover by 2 inches. Tie the thyme and parsley sprigs into a bundle. Pierce the unpeeled onion half with the cloves.
Add the herb bundle, cloved onion, reserved fennel fronds and stalks, and the 6 halved garlic cloves to the pot.
Bring to a boil, then reduce the heat to medium-low.
Simmer, partially covered, for 35 to 40 minutes, or until the beans are just tender.
Drain the beans, making sure to reserve the flavorful cooking liquid.
Discard the aromatics and wipe the Dutch oven clean.
Preheat your oven to 400°F (200°C).
In the clean Dutch oven, heat 2 tablespoons (30 ml / 1 fl oz) of olive oil over medium-high heat.
Add the diced carrots and fennel bulb, season with salt, cover, and cook for 10 minutes, stirring frequently, until they begin to brown.
Add the diced onion, season again with salt, and cook, covered, for another 6 to 8 minutes, stirring occasionally, until the onion is soft.
Stir in the 6 minced garlic cloves and cook for 1 minute more until fragrant.
Remove the pot from the heat and stir in the white wine vinegar, using a spatula to scrape up any browned bits from the bottom of the pot.
Gently stir in the cooked beans, the chopped thyme, 2 tablespoons (30 ml / 1 fl oz) of the chopped parsley, and ½ cup of the Parmesan cheese.
Pour in 1½ to 2 cups (480 ml) of the reserved bean cooking liquid—the liquid should come to just below the top of the beans.
Stir everything to combine.
In a small bowl, toss together the fresh breadcrumbs, the remaining 1 tablespoon (15 ml / 0.5 fl oz) of chopped parsley, and the remaining ¼ cup of Parmesan.
Drizzle with the final 1 tablespoon (15 ml / 0.5 fl oz) of olive oil and mix until the crumbs are evenly moistened.
Sprinkle the breadcrumb mixture evenly over the beans.
Bake, uncovered, for 40 to 45 minutes, until the topping is golden brown and the juices are bubbling thickly around the edges.
Let the gratin rest for at least 20 minutes before serving. This allows the beans to absorb the last of the savory juices, making the dish even creamier.
Letting the gratin rest for at least 20 minutes after baking is crucial. This allows the beans to absorb the flavorful liquid, resulting in a creamier, less watery dish.
While cannellini beans are classic, you can easily substitute other white beans like Great Northern or navy beans.
Adjust simmering time as needed.
