Trim the spring onions, carrot and celery. Roughly chop all the vegetables, then blitz in the food processor with the chillies (stalks removed). Add the sausages, 1 heaped teaspoon of fennel seeds and 1 teaspoon of oregano. Keep pulsing until well mixed, then spoon this mixture into a large frying pan on a high heat with a lug of olive oil, breaking it up and stirring as you go. Keep checking on it and stirring while you get on with other jobs.
Fill a large deep saucepan with boiling water. Season well then add the penne and cook according to packet instructions, with the lid askew.
Crush 4 unpeeled cloves of garlic into the sausage mixture and stir in 4 tablespoons of balsamic vinegar and the tinned tomatoes. Add a little of the starchy cooking water from the pasta to loosen if needed.
Drain the pasta, reserving about a wineglass worth of the cooking water. Tip the pasta into the pan of sauce and give it a gentle stir, adding enough of the cooking water to bring it to a silky consistency. Taste, correct the seasoning, then tip into a large serving bowl and take straight to the table with the Parmesan for grating over. Scatter over a few basil leaves.