Heat the 15 ml butter in a heavy-based frying pan.
Add the onions, season and sauté over gentle heat for about 20 minutes, stirring occasionally, until translucent.
Add the sugar, stir to dissolve and then add the vinegar.
Allow to simmer for a further 5 minutes.
Brush the beef fillet with the extra melted butter, sprinkle the herbs over and cover tightly with the bacon.
Place meat on the rack of an oven roasting pan and roast in a preheated oven at 160°C.
Slice the beef, spoon the onions onto a platter and top with the meat.
Serve the salsa verde on the side.