Preheat the oven to 220ºC/fan 200ºC/gas mark 7. Whisk the flour with the eggs and milk.
Put 1 tbsp oil in a roasting tin and heat in the oven for 1 minute. Add the sausages and brown in the oven for 20 minutes. Pour over the batter and arrange the sausages. Bake in the oven for 25 minutes.
Heat the remaining oil in a pan over a medium heat. Add the onion and cook for 10 minutes until golden. Stir in 1 tbsp flour and cook for 1 minute. Remove from the heat and stir in the stock. Return to the heat and bring to the boil, stirring until thickened. Stir in the mustard.
Boil the carrots for 8 minutes until almost tender, adding the peas for the final 2-3 minutes. Serve the toad in the hole with the peas, carrots and onion gravy.