Place the turkey breast in a 6-quart (6 L) stockpot and add enough cold water to cover the breast by about 1 inch (3 cm).
Remove the turkey to a platter, then add the halved onion (studded with cloves), chopped carrots, bay leaves, coarse sea salt, whole peppercorns, peeled ginger knob, garlic halves, and the 6 tablespoons white vinegar to the pot.
Bring the poaching liquid to a boil over high heat, then carefully lower the turkey back into the pot.
Reduce the heat to a bare simmer, cover, and poach the turkey for about 1 hour and 15 minutes, until cooked through.
Remove the pot from the heat and let the turkey cool in the poaching liquid, uncovered, for 30 minutes.
Drain and discard the poaching liquid and solids, then transfer the turkey to a platter to cool enough to handle.
In a large bowl, whisk together the grated lemon zest, 1/2 cup freshly squeezed lemon juice, 1 cup extra-virgin olive oil, and 2 tablespoons Dijon mustard until emulsified.
Stir the thinly sliced spring onions, sliced cornichons, and drained capers into the dressing.
Place the cooled turkey in a sturdy resealable plastic bag or a shallow container and pour the dressing mixture over it. Seal and turn to coat evenly.
Refrigerate the marinating turkey for at least 4 hours and up to 24 hours, turning the bag occasionally to redistribute the dressing.
At serving time, remove the turkey from the bag and slice crosswise into thin slices with a very sharp chef’s knife.
Moisten the slices with any remaining dressing, garnish with the chiffonade of fresh mint, and serve.
