Tip flour into the large mixing bowl and stir in the mustard powder with a good pinch of salt. Make a well in the centre, crack in the egg and pour in a dribble of milk. Stir with a wooden spoon, gradually incorporating some of the flour, until you have a smooth batter in the well. Now add a bit more milk and continue stirring until all the milk and flour has been mixedtogether. Stir in the thyme.
Put the sausages in a roasting tin. Add 1 tbsp of the oil, tossing the sausages in it to thoroughly coat the base of the tin. Roast for 15 minutes.
Take the hot tin from the oven, then quickly pour in the batter. It should sizzle a little when it first touches the hot fat. Put the tin back into the oven and bake for 40 minutes until the batter is cooked through, well risen and crisp. The middle should be set, not sticky or runny.
Soften the onions with the remaining oil in a large non stick frying pan for about 20 minutes, stirring often until they are golden brown. Sprinkle in the sugar for the final 5 minutes. Add the spoonful of flour, then cook stirring constantly for 2 minutes, coating the onions. Gradually pour in the stock, stirring well to make a smooth sauce. Bubble for 4-5 minutes to thicken, then season. Cut the toad in the hole into large wedges and serve with the gravy spooned over.