Preheat the oven to 220ºC, 200ºC fan, gas 7. To make the batter, whisk 125g flour with the eggs and milk.
Put 1 tablespoon of oil in a roasting tin and heat in the oven for 1 minute. Add the frozen sausages and brown in the oven for 20 minutes. Pour over the batter and arrange the sausages. Bake in the oven for 25 minutes.
Meanwhile, to make the onion gravy, heat the remaining 2 tablespoons of oil in a pan over a medium heat. Add the onion and cook for about 10 minutes until golden. Stir in 1 tablespoon of 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 about 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.