Measure 4 quarts of water and bring to a boil. Add asparagus stalks and 2 tablespoons salt. Set up ice bath next to the pot. Blanch asparagus for 1 minute, place in an ice bath and keep the asparagus cooking water until later. In the meantime, chop fennel into 1/2-inch batons and place in a 12 to 14-inch fry pan with 3-inch sides. Place over medium heat with olive oil and onion. Cook over medium heat until softened but not browned. Add rice and, stirring constantly, cook until rice turns opaque, about 2 minutes. Add asparagus cooking water until it covers the rice. Turn the heat up to high, stirring constantly, and cook the rice until the liquid level goes below the level of the rice. Continue adding the warm asparagus cooking liquid, 1 ladle at a time, to maintain the liquid level above the rice for 15 minutes. Drain asparagus and chop into 1-inch pieces. Add asparagus to rice and cook until rice is soft but still al dente. Remove from heat, add butter, Parmigiano and parsley and stir through. Serve immediately.