Heat the oven to 225°C/440°C, put the oil in a baking dish and place in the oven.
Whisk the eggs, milk, salt, mustard and thyme together then add the flour gradually. Leave the batter to rest for at least 15 mins.
Meanwhile take the baking dish out of the oven and add the sausages. Cook them for 5 minutes on each side.
Pour the batter on top of the sausages then cook for 20-25 mins or until risen and golden.
For the gravy, melt the butter over a low heat and cook red onion slowly until fragrant and soft. Add the flour and stir until mixture is dry. Add the stock gradually, stirring, then add the port. Bring to a boil then simmer for 10-15 minutes to thicken.
Nutrition information is an estimate and will vary depending on the exact ingredients used.