Melt a knob of butter in a pan. Add sliced shallots, crushed garlic and thyme. Cook on a high heat for a few minutes, then add the wine and the sugar. Boil until reduced to half, and then discard the thyme. Take the sauce off the heat and whisk in the mustard and the cold butter, a piece at a time. Add the balsamic vinegar and season with salt and pepper to taste. Barbecue or grill the sausages until nicely coloured and cooked through. Serve on a bed of mash with the red wine sauce and crusty bread on the side.