Heat the oven to 200c/fan 180c/gas 6. Put the sausages and onion wedges in an ovenproof baking dish. Drizzle over 2 tbsp oil, season then cook for 20 minutes until everything is browned a little.
While the sausages are cooking, put the plain flour and English mustard powder in a bowl and season. Gradually whisk in the eggs and milk to make a batter. Pour into the hot dish with the sausages then cook for another 30-40 minutes until crisp and golden.