Put the flour in a bowl and crack in the eggs. Whisk them together until fully combined. Gradually add the milk, whisking all the time, until all of the milk has been incorporated and you have a thick batter. Add a pinch of salt and pepper and leave to rest for at least 30 minutes.
Preheat the oven to 220 degrees C.
Put the sausages in an ovenproof dish and pour over the oil. Cook for 15 minutes until golden.
Remove from the oven and quickly pour over the batter. Return to the oven for 30 minutes until puffed up and golden.
Meanwhile, to make the gravy, peel and finely slice the onions. Melt the butter over a low heat, add the onions and cook gently for 10 minutes until soft.
Stir in the flour, then gradually add the stock, stirring all the time, until you have a thick gravy. Add the bay leaf and thyme and simmer for another 10 minutes.
To serve, dish up sausages and batter onto plates with a big pile of peas, and pour over the gravy.