Cut the base off of the fennel bulbs and cut a cone shape into the base to remove the core. Slice the fennel vertically (lengthwise) into 1/2 cm thick slices.
Melt butter in a heavy-bottomed stock pot over medium heat. Cook onions and fennel in butter for 2 minutes. Stir in the rice; cook for another 2 minutes, stirring constantly until lightly toasted. Stir in 1 cup vegetable stock; continue stirring until liquid is almost completely absorbed. Repeat this process with remaining stock, stirring constantly. Incorporating the stock should take 15 to 20 minutes in all.
Stir in cream, 1/4 cup Parmesan cheese and parsley and cook until rice is done and risotto is thick and creamy. Season to taste with black pepper. Divide risotto among 6 bowls and sprinkle with remaining Parmesan cheese.