Finely chop the pickles (or use relish), shallots, and dill. Add to a small bowl with mayonnaise and remaining ingredients. Mix well.
Taste and adjust seasoning to taste with salt and pepper, or add lemon juice or white wine vinegar, as needed.
Boil the fish:
In a saucepan, bring the milk, bay leaves, peppercorns and salt to a boil. Lower heat to a gentle simmer and add the fish. Cover and cook for 10 minutes, or until the fish is just cooked. Remove the fish from the milk and leave to cool.
Make the fish cakes:
Gently flake the fish into large bite size chunks, set aside. [Tip: you can use your fingers or you can use 2 forks for the flaking.]
Add mashed potatoes (or use leftovers) in a large bowl with the spring onions, lemon zest, parsley, dijon mustard, 1 tsp salt and a generous pinch of black pepper, and mix until well combined. Season to taste with more salt and pepper if needed.
Add ½ of the flaked fish and beat it firmly into the potato mixture using a hand mixer. Add the rest of the fish and gently stir, leaving “large flakes” of the fish. Place the mixture in the fridge for 15 minutes.
Remove from the fridge, and with slightly dampened hands take a handful of the mixture and form it into a thick patty. The size should be roughly 10cm in diameter and about 2 cm thick (or 4" x 1 ½" thick) but you can make them smaller if you wish. Repeat with the remaining mixture. You should end up with 5-6 cakes.
Next, place each fish cake first into the flour (with salt and pepper), then the egg/milk, and lastly in the mixture of breadcrumbs. TIP: The fish cakes can be prepared up to this stage in advance and refrigerated until ready to fry.
Before frying, bring the fish cakes back to room temp and heat a large non stick frying pan over medium heat. Add the butter and oil and when hot place the fish cakes gently in the pan. Let them cook for a few minutes before flipping, making sure the underside of the cakes and are golden brown; then turn them over (about 6-7 min total). Cook the other side for another few minutes until also golden. AIR FRY: You can also air fry the fish cakes at 360 degrees for 15-20 minutes.
Serving:
Remove fish cakes from the pan and let them rest for 5 minutes before serving with the dipping sauce. Also add a few lemon wedges for serving, and sprinkle with chopped green onions.