Lightly score the skin of the duck breasts, then generously season with the peppercorns and a sprinkling of salt. Lay the duck breasts, skin-side down, in a non-stick sauté pan, then place over a low heat. Leave the duck for 15 mins to brown and release its fat, then flip over onto the flesh side for 5 mins.
Remove the duck from the pan, then turn up the heat. Add the potatoes to the pan, fry until brown and crisp, then scatter over the parsley and garlic. Scoop out with a slotted spoon onto a plate, then season with salt.
Keep the pan on the heat. Fry the bacon until crisp, then add the cabbage. Cook for 1 min, add a splash of water, then fry for 2 mins, just until the cabbage is wilted. While the cabbage is cooking, whisk any juices from the duck with the vinegar and olive oil. To serve, carve the duck breast into slices. Fan out on large dinner plates, spoon a neat bundle of cabbage on one side, then pile a serving of potatoes on the other. Drizzle over the dressing and serve.