Cut the fish fillets into small cubes or pulse them quickly in a food processor, just until roughly chopped – you want some texture, not a smooth paste.
In a large bowl, combine the chopped fish, mashed potatoes, onion, garlic, egg, the 2 tablespoons of breadcrumbs and the chopped herbs. Season generously with salt and pepper and mix lightly.
Use your hand to mix everything until well combined. Divide the mixture into 9 equal portions, shape each into a ball, then gently flatten into a thick disc.
Coat each patty in breadcrumbs on both sides, pressing gently so the crunchy layer sticks well.
Heat a generous layer of oil in a wide skillet over medium heat. Fry the patties for about 4–5 minutes per side, until deep golden-brown and crispy.
Transfer the patties to a plate lined with paper towels to drain any excess oil. Serve hot with the yogurt sauce and lemon wedges.
In a small bowl, stir together the Greek yogurt or sour cream, mayonnaise, chopped dill and a squeeze of lemon juice. Season with a pinch of salt and pepper, then chill until serving.