Prepare the squash by halving it from top to bottom along the middle with a sharp knife. Hold it steady with one hand and remove the skin with a vegetable peeler, before scooping out the seeds with a spoon.
Peel the onions and cut them into quarters, separating the segments.
Chop half the sage leaves into pieces and set aside the rest for later.
Preheat the oven to Gas Mark 7 / 220 C / 200 Fan.
Scatter the squash, onion wedges and chopped sage in a large non-stick roasting pan that is big enough to take them in a single layer.
Drizzle over 2 tablespoons of the oil and toss everything to coat it in the oil. You can use a flavoured oil if you wish.
Cut the sausages into bite-size chunks and tuck them in amongst the vegetables, then brush them with the remaining oil.
Roast for 20 minutes, then give everything a quick stir and roast for a further 10-15 minutes, or until the sausages are brown and the vegetables are turning golden at the edges.
Meanwhile, heat a little oil in a small frying pan (enough to just cover the bottom of it). When the oil is hot, drop in the remaining dry sage leaves and fry for a few seconds until just crisp. Remove with a slotted spoon and drain on kitchen paper.
Scatter the fried sage leaves over the roasted sausages and vegetables. Serve with green vegetables of your choice.