Peel, wash, and boil the potatoes in salted water over medium-high heat for 20 minutes until tender.
Peel the onion and cut into cubes. Wash the dill, parsley, and chives, pat dry, and chop. Set aside a small portion of the chopped herbs for garnish. Clean the apple, quarter it, remove the core, dice it, and drizzle with apple cider vinegar.
In a bowl, mix the yogurt with the diced apple, chopped herbs, onion cubes, salt, and pepper. Refrigerate until ready to serve.
Rinse the cod fillets and pat dry. Season with salt and pepper. Heat the oil in a large skillet over medium-high heat and fry the fillets for 4 minutes total until the fish flakes easily with a fork.
Drain the potatoes and place on a serving plate with the fish. Spoon the yogurt sauce alongside and garnish with the reserved herbs.