Add a splash of olive oil to a hot pan and fry the chopped onion until translucent, then stir in the garlic and cook for a further minute. Set aside to cool.
Trim away the silver skin and cut off the tail end of the fillet so both ends have an equal thickness (cook this end bit for a sneaky chef’s snack or save it for later).
To form the stuffing pocket in the fillet, slowly slice lengthways down its centre taking care to not cut all the way through the meat.
Season the fillet well (including the pocket) with salt and freshly ground black pepper.
Fill the pocket with the butter, cooked onions, exotic mushrooms and thyme leaves and tightly tie about 6 pieces of butcher’s string around the fillet to secure the stuffing.
Grill the fillet over a very hot fire, searing it on all sides for about 2 minutes per side. Then move the fillet over to slightly cooler coals and braai for 20-25 minutes, turning it occasionally to evenly cook the beef to medium-rare. A thick piece of meat may take a little longer but keep your eye on it so it doesn’t over cook, medium-rare is recommended. Brush with olive oil if necessary.
Leave the meat to rest for 5 to 10 minutes before slicing it into thick rounds. Remember to remove the string before serving.
These heavenly stuffed medallions of beef can be served with any side dishes of your choice. Crispy roast potatoes and a fresh salad would be delicious accompaniments to finish off this meal.