Allow the fish to thaw at room temperature for about 1 1/2 hours. Wash, peel and slice the quinces and cook them in boiling salt-lemon water for about 10 minutes. Clean and wash spring onions and cut them into pieces.
Wash the fish and dab dry. Bring 1/4 litre of water, vinegar, 1 pinch of sugar, salt, laurel and juniper berries to the boil. Add the fish and let it simmer at low heat for about 12 minutes. In the meantime, melt the remaining sugar in a dry pan and add 1/4 litre of water while stirring.
Season to taste with salt and pepper. Drain the quinces, add them to the caramel and simmer for about 5 minutes. Heat the oil in a pan, fry the spring onions for about 2 minutes, turning them, add them to the caramel and let them simmer for a while.
Arrange vegetables and fish on a plate, sprinkled with pepper. Serve garnished with lemon wheels and dill.