Fennel Risotto

In a medium saucepan, bring the chicken broth to a simmer; keep warm. In an enameled medium cast-iron casserole, melt 4 tablespoons of the butter over moderately high heat. Add the onion and fennel, season with salt and cook over moderately high heat, stirring, until softened, about 10 minutes. Add the rice and cook for 1 minute, stirring constantly to coat the rice with butter. Add 1 cup of the warm stock and cook over moderately high heat, stirring constantly, until nearly absorbed. Continue adding the stock 1 cup at a time and stirring constantly until it is nearly absorbed between additions. The risotto is done when the rice is al dente and suspended in a thick, creamy sauce, about 20 minutes total. Stir in the remaining 2 tablespoons of butter, the 1 1/2 cups of cheese and the 2 tablespoons of fennel fronds. Garnish with more fennel fronds and serve immediately, passing grated cheese at the table.