Preheat the oven to 450 degrees. Butter four 4-inch round gratin dishes.
Bring 4 quarts of water to a boil in a large pot, and add 2 tablespoons salt. Add the fennel and blanch until tender, 8 to 10 minutes. Drain thoroughly.
In a medium bowl, mix the fennel with the besciamella and Fontina. Divide among the gratin dishes, and pat down with the back of a spoon. Bake in the upper third of the oven for 25 minutes, or until bubbling and hot.
Place a 2-ounce dollop or square of robiola. Bake for 5 to 6 more minutes, until the robiola is hot and soft and the crumbs have melted into it. Allow to stand 3 minutes before serving.