Place the turkey breast in the stockpot and add enough cold water to cover by 1 inch. Remove the turkey to a platter. Add the onion, carrots, bay leaves, salt, peppercorns, ginger, garlic, and vinegar to the pot. Bring to a boil over high heat. Carefully lower the turkey into the pot, reduce the heat to a bare simmer, and poach, covered, for 1 1/4 hours.
Remove the pot from the heat and let the turkey cool in the liquid, uncovered, for 30 minutes.
Drain the turkey and discard the poaching liquid and solids.
Prepare the marinade: In a bowl, whisk together the lemon zest, juice, oil, and mustard. Stir in the spring onions, cornichons, and capers.
Place the turkey in a sturdy resealable plastic bag and pour the marinade into the bag. Seal the bag and turn it back and forth to coat the turkey. Refrigerate for at least 4 hours and up to 24 hours.
At serving time, remove the turkey from the bag, reserving the marinade, and place it on a cutting board. With a meat slicer or a very sharp chef’s knife, cut the turkey into paper- thin slices. Arrange the turkey slices on a platter. Moisten the turkey with the marinade. Garnish with the mint, and serve.