Preheat the oven to 170°C/ 325F.
First, make the red wine sauce. In a small pan, heat the vinegar and red wine. Boil until the sauce has reduced by half and then add the sugar, stock and thyme and leave to reduce until thickened. Sieve the sauce and season.
In another pan, melt the butter gently and cook the spinach until wilted. Season to taste, drain any excess liquid.
Increase the oven temperature to 180°C/ 350F, Gas mark 4 and prepare the lamb. Blend the breadcrumbs with the fresh herbs in the food processor for 2-3 minutes.
Brush the lamb with the mustard and then press the herb crumbs onto the mustard side of the lamb.
Heat the olive oil in a frying pan and sear the lamb on all sides for about 2 minutes. Place in a roasting dish and put in the oven for about 20-22 minutes or less if you like it pink in the middle.
Remove from the oven and leave to rest in a warm place for about 10 minutes.
To serve, arrange the wilted spinach on each plate, carve the lamb into cutlets and arrange on top of the spinach. Drizzle with a little of the red wine sauce. Serve with potato gratin or roast potatoes.