Trim the top off the aubergine and dice it into 1cm chunks. Warm a deep frying pan or wok for 1 min over a medium heat. Add ½ tbsp oil and the aubergine. Season. Cook for 6-8 mins, stirring a few times, till the aubergine is browned all over.
While the aubergine browns, slice 3 sausages open and squeeze the sausagemeat into a bowl. Discard the skins. (see below for our tip on what to do with your leftover sausages).
Lift the aubergines out of the pan with a slotted spoon and drop them into a clean bowl. Add another ½ tbsp oil to the pan and add the sausagemeat. Set the heat to medium-low. Fry for 5 mins, stirring now and then, till the sausagemeat browns a little.
While the sausagemeat browns, peel and finely chop the onion. Trim the dry ends off the celery and finely chop it.
Stir the onion and celery into the sausagemeat. Add a splash of water. Season with a little salt and pepper. Cook for 5 mins, stirring now and then, till the veg look glossy.
Peel and crush or grate the garlic. Finely chop the parsley stalks (keep the leaves for later). Stir in the garlic and parsley stalks. Add 1 tsp fennel seeds. Return the aubergine to the pan. Tip in the tomatoes. Cover, turn up the heat and bring to the boil. Turn the heat down to medium-low and simmer for 20 mins till the sauce looks rich and the veg are tender. Stir every so often.
Put a large pan of salted water on to boil. When it’s boiling, add the penne. Simmer for 8 mins till the pasta is cooked but not too soft. Drain, and add to the pasta sauce. Toss to coat in the sauce. Serve in warm bowls, garnished with the parsley leaves.