Place the turkey breast in a 6-quart (6l) stock pot and add enough cold water to cover by 1 inch (3cm). 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 heat to a bare simmer, and poach, covered, for 1 1/4 hours.
Remove the pot from the heat and cool the turkey in the liquid, uncovered, for 30 minutes.
Drain the turkey and discard the poaching liquid and solids.
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 marinated into the bag. Seal the bag and turn it back and forth to evenly coat the turkey with the marinade. Refrigerate for at least 4 hours and up to 24 hours.
At serving time, remove the turkey from the bag and place on a cutting board. With a very sharp chef’s knife, slice the turkey crosswise into thin slices. Moisten the turkey slices with the marinade. Garnish with the mint and serve.