Preheat the oven to 200 C Gas 6. Peel the parsnips and quarter them from root to tip. If they have coarse woody cores cut them out.
Peel the onions but leave the root end intact. Cut each onion from root to tip into quarters or eighths, depending on the sizr.
Place the parsnips and onions in a roasting tin, trickle over the oil, sprinkle with the garam masala and season well with salt and pepper.
Cover the tin tightly with foil and cook in the oven for 30-40 minutes until the onions are tender. Meanwhile, lightly brown the sausages in a frying pan over a fairly high heat for 3-4 minutes.
Take the roasting tin from the oven and remove the foil. Add the sausages to the tin and dot the butter around. Sprinkle with the herbs if using.
Return to the oven and roast uncovered for about 20 minutes until the sausages are cooked and everything is golden and starting to caramelise.
Allow to rest in a warm place for 5-10 minutes before serving with a green salad.