Cook the potatoes in salted boiling water until tender (about 15 minutes). Add the peas for the last few minutes. Drain then return to the pan and lightly mash with the butter (I prefer to leave the potato a bit ‘chunky’). Tip into a large bowl.
Meanwhile, put the salmon in a microwavable dish with 100ml milk, then cover with plastic wrap and microwave for 2 minutes or until just cooked (a thicker piece of fish might need a minute or 2 longer).
Flake the salmon into the bowl with the potato, then add the lemon zest, mustard, fresh herbs, cheese, chili flakes, and mayonnaise. Add a generous grinding of salt and pepper and mix well.
Cool the mixture for a while in the fridge (half an hour is fine), then take golfball-sized amounts of mixture and shape into 8 to 10 fish cakes.
To coat the fish cakes, put the flour, egg and breadcrumbs in separate dishes, then roll the fishcakes first in the flour, then in the egg and lastly in the breadcrumbs. Place on a plate or baking tray ready to cook.
Heat up a good drizzle of oil in a frying pan, then pan fry the fishcakes for about 5 minutes on each side or until golden and crispy. Serve immediately.