In a medium pot, bring rosé wine to a boil and let simmer for 3 minutes. Meanwhile, in a clean jar, neatly arrange the fish, bay leaves, onion and rosemary sprigs.
Pour over the rosé wine. Add vinegar and peppercorns, and close the lid tightly.
Place the jar in a pot and fill the pot with boiling water until the water goes 2/3 up to the rim of the jar, and leave for 15 to 20 minutes while the water simmers gently. Leave to cool.