I don't use sausages, but buy sausage meat, mix it with some herbs (nothing to stop you skinning sausages and mashing them up) and fried chopped up onion, which I then heat to loosen the fat and make it easier to mould into a long sausage shape. I find it easier to make a sausage roll about 12" long and then cut before cooking. I suppose I would have fried the sausage meat for three or four minutes, so par-cooked, but par-cooked the whole way through.
I use chipolatas and then remove the skin. Pop them raw on ready rolled puff pastry and roll up. Cut into whatever size you want, brush with beaten egg and in the oven for 20 mins. Delicious, hassle free home made sausage rolls. The dcs can't get enough of them and they're fab as an alternative packed lunch.