In a large skillet over medium heat, saute the onion in the butter. When it is golden, add the wine and let the liquid cook off. Remove the sausage from its casing and crumble onto the onions. Cook thoroughly, mashing with a fork. After a few minutes add the sage, salt and pepper.
Meanwhile, bring a large pot of salted water to a boil. Cook the cavatelli until al dente, and drain it, reserving a cup of the cooking liquid. Add the cavatelli to the sausage mixture and cook together, adding the reserved cooking water, for a minute or two. Serve immediately with a sprinkle of Parmigiano.