Fill a pot for the pasta with about 6 quarts of water, place over high heat, and bring to a boil.
Peel and finely chop the onion. Put the butter in a 12-inch skillet, add the chopped onion, and place over medium-high heat. Sauté until the onion
turns a rich golden color, about 5 minutes.
While the onion is sautéing, peel the tomatoes and coarsely chop them. Remove the sausage from the casing.
When the onions are ready, add the sausage and break it up into small pieces with a wooden spoon. Cook until it begins to brown, 2 to 3 minutes.
Add the tomatoes and season with salt and pepper. Lower the heat to medium. After 5 minutes, coarsely chop the basil leaves and add them to the sauce. Continue cooking until most of the liquid the tomatoes release has evaporated, about 5 more minutes.
Add about 2 tablespoons salt to the boiling pasta water, add the fusilli, and stir well. Cook until al dente.
When the pasta is almost ready, stir the ricotta into the sauce over medium heat for about 1 minute, then remove from the heat. When the pasta is done, drain well, toss with the sauce and the grated Parmigiano-Reggiano, and serve at once.