Heat the grill to medium-high heat. Grill the corn until lightly charred on all sides, about 10 minutes. Remove the kernels from the cobs and reserve the cobs.
Break the reserved corn cobs in half and place them in a large pot with the stock. Bring to a simmer over medium-high heat, and then turn off the heat and steep the cobs in the stock for 30 minutes.
Melt 2 tablespoons of the butter in a saute pan over medium heat. Add the fennel and cook until soft, about 10 minutes. Remove the fennel from the pan and set aside. Add another 2 tablespoons of the butter to the pan and cook the corn kernels until soft and velvety, 10 to 15 minutes. Season the corn with salt and pepper, and set it aside.
Pass the corn stock through a strainer into a saucepan and discard the cobs. Keep warm over low heat. Heat the oil in a high-sided skillet over medium heat. Add the onions and saute, stirring occasionally, until starting to soften, about 3 minutes. Add the rice and cook, stirring frequently with a wooden spoon, until it no longer appears chalky on the outside, 4 to 5 minutes.
Add the wine and simmer, stirring frequently, until absorbed, about 5 minutes. Add 1 1/2 cups of the corn stock, stirring frequently until the liquid is absorbed, about 7 minutes. Add another 1 1/2 cups of stock and simmer, stirring frequently, until the liquid is mostly absorbed and the rice appears fluffy, but is still slightly firm. If the rice is still hard or chewy, add the remaining stock, 1 cup at a time, and keep stirring until the proper doneness is reached. The total cooking time should be 20 to 25 minutes. You might not use all 6 cups of stock, but any remaining stock can be refrigerated and used for another dish.
Stir in the reserved fennel and corn kernels, the remaining 2 tablespoons of butter and the Parmigiano-Reggiano. Season with salt and pepper, and serve. Garnish with fennel fronds.