Sweat the onion in butter (or butter and oil) in a large pot. Once the onion is soft and translucent, add the asparagus rounds and sauté them gently so they absorb the flavor of the onion-scented butter.
Add the rice and let it toast until it turns a chalky white color and begins to "crackle". Add a splash of white wine to the pot, stir and let the wine evaporate completely.
Add the asparagus broth one ladleful at a time, stirring it into the rice and letting it absorb the broth before adding another ladleful, until the rice is al dente. If you run out of broth, continue with water. This process usually takes about 20 minutes.
While the risotto is cooking, melt a good knob of butter in a skillet and add the asparagus tips. Sauté them over moderate heat for a few minutes, until fully tender, then set aside.
When your rice is done, take it off heat. Add the parmigiano-reggiano and, if using, a nice chunk of the soft cheese. Stir vigorously until the cheese has melted into the rice and the risotto is nice and creamy, perhaps about a minute or so.
Serve immediately, with a few of the aspargus tips on each plate for garnish.