Sift the flour and salt through a sieve (strainer) into a mixing bowl and make an indent in the centre. Break the egg into the indent and stir in half of the milk, gradually working the flour down from the sides. Beat this mixture vigorously until it is smooth and bubbly, then stir in the remainder of the milk. Rest the mix for at least 30 minutes. While it's resting, preheat your oven to very hot (Mk 9 - 475ºF - 240ºC).
Meanwhile, peel and slice the onion, roughly crumble the Gorgonzola and roughly chop the rosemary. Pour enough oil to cover the base of a large roasting tin or oven-proof dish. Heat in the oven and then add the sausages, red onion and lardons and cook until browned and the oil is smoking hot (no more than 5-10 minutes).
Remove the tin from the oven and, as quickly as possible, pour the batter mix around the sausages. Dot the whole thing with the crumbled Gorgonzola and chopped rosemary then carefully return the whole dish to the oven and cook for 25-30 minutes until risen, crisp and golden. (If you poke the tip of a knife into the batter in the middle of the tray it should be set, not sticky or runny.) Serve immediately with Colcannon and a glass of beer or cider.