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Whisk the flour, eggs and milk together with a pinch of salt until smooth. Cover and rest for 30 mins (or chill in the fridge overnight).
Heat oven to 220C/200C fan/ gas 7 and put a large pan of salted water on to boil. Heat the oil in a large roasting tin (40cm x 30cm approx) in the oven for 10 mins. Once hot, pour in the batter and cook for 25 mins or until puffed up and golden.
Meanwhile, tip the peas and kale into the boiling water and cook for 2-3 mins until tender. Drain well and season.
When the pudding comes out of the oven, remove from the tin, flatten with the back of a large metal spoon and halve lengthways. Spread 1 tbsp horseradish over each half, followed by the mustard, beef slices, peas, kale and gravy. Wrap up and serve.