Simmer ham hocks with vegetables, peppercorns, salt, and herbs in water for two hours.
Strain the cooking liquid and reserve 250ml.
Steam spring onions for five minutes until tender and allow to cool.
Soak gelatine leaves in cold water for five minutes until pliable.
Stir soaked gelatine into the simmering reserved cooking liquor until dissolved.
Shed ham from bones, cut into 2.5cm slices, and place half in a lined terrine mould.
Layer spring onions over the ham and top with the remaining ham slices.
Pour the gelatine mixture over the ham and tap the mould to remove air.
Cover the terrine, weigh it down with tins, and refrigerate for two hours.
Boil vinegar and sugar together then remove from heat.
Stir fry mustard seeds and chilli flakes in olive oil for one minute.
Fry marrow rounds for 3 minutes until softened and season to taste.
Combine marrow with the vinegar mixture and chill in the fridge.
Slice the terrine and serve with a spoonful of drained marrow.