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Tip the flour and eggs into a large bowl, season and whisk to a thick, smooth paste. Stir in the milk until the batter is the consistency of single cream, then stir in 2 tsp mustard. Leave to rest for 30 mins.
Heat the oven to 240C/220C fan/gas 9. Pour 2 tbsp oil into a heavy roasting tin, casserole dish or ovenproof skillet pan and tip to fully coat with the oil, then put on the middle shelf of the oven for 10 mins. Add the sausages, well spaced apart, and cook for 5-7 mins, then add the broccoli and sliced leek, and cook for another 3 mins. Mash the cheese with the 1 tbsp mustard.
Pour the batter over the sausages, broccoli and leeks, then scatter over most of the mustardy cheese. Cook for 15 mins before turning the oven down to 220C/200C fan/gas 7 (don’t open the oven door during this time). Sprinkle over the remaining mustardy cheese, then cook for 5-8 mins until melted, and the batter is puffed and golden brown.
Meanwhile, make the gravy. Fry the onion with the remaining oil for 10-15 mins until lightly golden, adding a little water if they catch. Stir in the flour until it disappears, then add the vinegar and thyme. Season well. Sizzle until the vinegar evaporates, then pour in the stock and bubble uncovered for 10 mins, or until the gravy is the thickness you prefer.