Start by preparing the vanilla syrup; place the sugar and water in a small saucepan and bring to a simmer over a moderate heat, stirring to help dissolve the sugar. Reduce the heat and simmer for 1 minute until slightly thickened. Remove from the heat and stir in the vanilla extract, then allow the syrup to cool to room temperature.
Pre-heat the grill to hot and arrange the black pudding on a lined baking tray. Grill for 2-3 minutes on both sides until crispy on the outside.
Meanwhile, melt the butter in a frying pan set over a moderate heat and saute the apple slices for 3-4 minutes, tossing occasionally. Season and set to one side. Remove the black pudding once ready and roughly chop.
Divide the watercress evenly between 4 serving plates and top with apple slices and the chopped black pudding. Drizzle the vanilla syrup on top and around the salad. Serve immediately.
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