Clean and thinly slice the onions, and put them in a heavy-bottomed frying pan with the olive oil. Put the lid on the pan and cook over a very low heat for a couple of minutes. Pour in 2-3 spoonfuls of water and continue cooking, without putting the lid back on, until the onion is translucent. Skin the sausages and break them into pieces by hand; put them into the frying pan, stir and sauté while breaking them up into small pieces with the tip of a wooden spoon. Wash and dry the grapes, cut them in half and remove the seeds if you don’t like them, otherwise you can leave them. Add the grapes to the pan with the sausage, pour in the balsamic vinegar, season with salt and pepper, stir well and leave to cook over a lively heat for a few minutes. When the vinegar has evaporated, turn off the heat and put the lid on the pan. Cook the Tubettini in plenty of boiling, salted water, strain when “al dente” and toss them in the frying pan with the hot sauce for 1 or 2 minutes. When everything is amalgamated, serve immediately with a sprinkling of grated pecorino, if you wish, and some freshly ground pepper.