1. Place cod in a large bowl and cover generously with water. Set in fridge to soak for 24 hours, changing the soaking water once during the first few hours. 
2. Place water, onion, thyme, rosemary, and cod in a large saucepan over high heat and bring to a boil. Reduce heat to low and simmer until cod breaks apart into pieces and becomes tender, about 25 minutes.
3. Meanwhile, cook potatoes in boiling water until tender, about 15 minutes. Drain and mash. Set aside.
4. Drain the fish mixture. Remove and discard the onion and herbs, and shred the cod with a fork. Set aside.
5. Mix together the mashed potatoes, 1 beaten egg, pepper, garlic, lemon zest and juice, and shredded cod in a bowl.
6. Shape mixture into 18 balls. Roll them in flour, then in 2 remaining beaten eggs, and finally in the panko breakcrumbs.
7. Heat half the oil in a large skillet over high heat and fry half the croquettes until golden brown on all sides, turning them at the halfway point, about 4 or 5 minutes. Set on paper towel to drain and repeat with remaining oil and croquette mixture.
8. Serve with a green vegetable and spicy mayonnaise or aioli.

Chef’s secret

You can use other white fish like haddock, turbot, or halibut for this recipe. If you use one of these, salt the fish to taste. (If using salt cod or salted pollock, you won't require any added salt.)

Nutrition tip

You can also opt to bake this recipe in the oven. Just place the croquettes on a parchment-lined baking sheet and bake for 20 minutes at 200°C (400°F) until golden brown, turning at the halfway point.