Preheat oven to 400°.
Toss fennel and unpeeled garlic cloves with 3 tablespoons of olive oil on a sheet pan. Season generously with salt and pepper and roast in the oven for 30–40 minutes, stirring occasionally during cooking.
Take out the pan and let it cool, then squeeze roasted garlic out of the skins and put aside.
In a small pan, heat ½ cup olive oil over medium heat. Add minced garlic cloves and cook until lightly golden. Add cannellini beans and rosemary and cook for 1 minute more. Remove from heat.
Transfer garlic bean mixture, fennel, roasted garlic, lemon juice, remaining ¼ cup olive oil and all but 3 tablespoons of cheese to a food processor. Purée until smooth.
Raise oven temperature to 450°. Transfer purée to a small baking dish, sprinkle with remaining cheese, and bake until golden on top, about 15–20 minutes. Serve hot with crostini.