Boil the potatoes until tender. At the same time, gently cook the onion in the butter till softened and caramelised. Drain the potatoes and crush them with the back of a fork.
Mix together the crushed potatoes, caramelised onion, ham hock, black pudding, parsley and celery salt. Season with Worcestershire sauce to taste.
Divide the mixture into four equal amounts, and using lightly floured hands, shape into patties. Chill in the fridge for 30 minutes.
Make a dressing by whisking together the olive and rapeseed oils, vinegar, mustard, sugar and lemon juice. Season to taste.
Heat 1 tbsp sunflower oil in a large, non-stick frying pan over medium heat. Fry the patties, in batches, for 2-3 minutes on each side, until crisp and golden on the outside and hot through to the middle. Add a little more oil as needed.
Keep the patties warm while you fry the eggs. Toss the pea shoots in the dressing.