3 tbsp vegetable, sunflower or rapeseed oil, for frying
Put the potatoes and sweet potatoes in a large pan of cold, salted water, cover and bring to the boil. Simmer for 10 mins until starting to soften around the edges but not completely tender. Add the broccoli and cook for 4 mins more, then add the peas and cook for another 30 secs. Drain and leave everything in the colander to cool completely. Can be kept in the fridge overnight.
Tip the veg and potatoes into a food processor. Add the parsley, lemon zest, mustard, mackerel, flour and some seasoning. Blend until the mixture is finely chopped and starts to clump together. Remove the blade from the mixer and shape into 8 cakes, dusting your hands and the cakes in flour. Put the cakes on a plate and chill for 30 mins or until they have firmed up, Can be wrapped and frozen for up to 2 months.
Heat the oil in a large non-stick frying pan. Cook the cakes for 5-8 mins each side until golden, crispy and hot throughout. Serve with extra peas and lemon wedges.