Trim away the silver skin on the fillet then rub the beef with olive oil and season well with salt and freshly ground black pepper.
Heat a large frying pan over a high heat, add a splash of olive oil and evenly brown the fillet on all sides, searing the ends to seal in the juices.
Remove the beef from the pan and coat well with the fresh chopped thyme and garlic.
Spread the sliced red onion across the base of a roasting dish and place the beef on top of the onions.
Spoon 2 tablespoons of butter along the top of the fillet, season with salt and pepper and roast in the oven for approximately 35 to 40 minutes for medium rare beef. Cooking time will depend on the thickness of the fillet.
Transfer the roast to a chopping board, cover lightly with tinfoil and allow to rest.
Pour the beef drippings and onions from the roast’s dish into a saucepan, add the 75g of butter and cook until the onions are golden and caramelised.
Pour in the beef stock, Worcestershire sauce and thyme, bring to the boil before reducing to a low heat and allow to simmer for 10 minutes.
Slice the beef tenderloin and spoon over the decadent French onion au jus and serve extra on the side when serving.