Preheat the oven to gas 6, 220°C, fan 200°C.
In a bowl, combine the flour, mustard powder, herbs and ½tsp salt. Beat the milk into the egg, then gradually pour it into the flour mixture, stirring continuously, until it creates a smooth batter. Transfer to a small jug for easy pouring.
Divide the oil between two 12-hole mini-muffin tins and put them in the oven for 8-10 minutes until sizzling hot. Remove from the oven and carefully pour the batter into the holes to half-way full. Return to the oven and cook for 8-10 minutes, until risen and crisp. Leave to cool.
In a bowl, mix the crème fraîche, horseradish, lemon zest and 1tsp of lemon juice and season. To serve, spoon a little horseradish mix onto each pudding, top with watercress.
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