Prep beef - Pat beef dry with paper towels, sprinkle with salt and pepper all over. Place the beef on a rack on a tray then refrigerate uncovered 12 to 24 hours. (Can skip, Note 5)

Butter Slather - Mix Butter Slather ingredients in a bowl.

De-chill - Remove the beef from the fridge 2 hours prior to cooking. This helps the beef cook evenly.

Sear - Heat oil in an ovenproof heavy based pan over high heat until smoking. (Note 6) Brown the surface of the beef all over, aggressively and quickly. Then put the beef back on the rack to cool for 15 minutes (so the butter doesn't instantly melt when you put it on).

Preheat oven to 120°C/250°F (both fan and standard ovens) (Note 7).

Butter - Put beef back in cooled skillet. Slather 3/4 of the butter onto the beef - top and sides (not underside).

Roast 40 - 50 minutes - Roast for 15 minutes. Remove, slather remaining butter onto the top and sides. Roast for a further 25 - 35 minutes or until the internal temperature is 53°C/127°F for medium rare (Note 8 for other levels of doneness).

Rest & serve - Remove beef from the skillet onto a cutting board and rest for 10 minutes. The internal temperature will rise 56-58°C / 133-136°F (this is medium rare). Slice thickly and serve with Creamy Mushroom Sauce!

Roasting juices - Pour all juices in skillet into a bowl or jug. The fat will rise to the surface. Pour off 1/4 cup (60ml) of the fat back into the skillet (reserve the rest for later).

Cook mushrooms - Turn the stove onto high. Once the butter is hot, add mushrooms and cook until they start to sweat. Then add thyme and garlic, and cook for a further 2 minutes until the mushrooms are softened.

Sauce - Add chicken stock/broth and reduce by half. Add marsala, cream, pepper and remaining reserved roasting juices. Simmer on medium / medium high for 5 minutes until it reduces by half and the sauce is thickened. Taste - it should be slightly on the salty side (which will seem just right with each mouthful of beef). Add more salt if needed. Serve with beef.