Trim any sinew off duck breasts. Gently score the skin of the duck breasts in a criss-cross fashion, making sure not to cut into the flesh. Sprinkle each duck breast with the Vintage Merlot Salt, with the majority of the salt being rubbed into the skin side.
Heat a large cast iron frying pan over medium-high heat, then add a tiny splash of olive oil, just enough to moisten the surface of the pan. Place the duck breasts skin side down in the pan, with plenty of space between them. Sear the duck until the skin is golden brown and most of the fat has rendered into the pan – about 5-8 minutes – then pour most of the fat from the pan (reserve it, this is perfect for roasting potatoes in later), flip the duck breasts and cook for a further 5 minutes, flesh side down.
Remove the duck from the pan and wrap loosely in foil. Allow to rest for about 8 minutes.
Again pour off the fat from the pan and return to the stove over high heat. Once hot, place the grapes in the pan and cook, turning occasionally, until they have softened and the skins blistered. Reduce the heat slightly and deglaze the pan with the vinegar, spooning the liquid over the grapes. Cook for about 1 minute, then remove from heat.
Slice duck breasts, pour any juices that have leaked out into the foil package back into the frying pan and mix with the vinegar liquid.
