For the pork fillet:
Preheat the oven to 180 °C. Toss the grapes with 2 sprigs of thyme, olive oil, salt and pepper. Roast in the oven for 10 minutes until just softened and starting to burst. Pat the pork fillet dry and brush all over with Dijon mustard. Season well. Lay the prosciutto slices slightly overlapping on a board. Place the fillet at one end and roll tightly to wrap. Trim the fillet and neatly tuck in the ends to make a parcel. Heat a frying pan over medium-high heat. Add half the butter, the remaining thyme and the rosemary, then sear the fillet on all sides until golden, about 5 minutes. Transfer to a baking tray, cover loosely with foil and roast for 20 minutes, or until just cooked through. Rest for 5 minutes before slicing. While the fillet is roasting, melt the remaining butter in a small saucepan over medium heat. Add the sage and fry until crisp and the butter turns nutty brown.
To serve:
Spread the mash onto a serving platter, arrange the sliced pork on top, spoon over the pan juices and scatter over the roasted grapes and fried sage leaves. Drizzle with any remaining sage-infused brown butter.
