Tip the miso paste, Sherry or sake, sugar and garlic into a sealable food bag. Season with a generous grinding of black pepper, then squash it all together until completely mixed. Add the steak, gently massage the marinade into the steak until completely coated, then seal the bag. Pop the bag into the fridge and leave for at least 1 hr, but up to 2 days is fine.
To cook, heat a heavy-based frying pan, griddle pan or barbecue until very hot. Wipe the excess marinade off the steak, then sear for 3 mins on each side for medium-rare or a few mins longer if you prefer the meat more cooked. Set aside for 1 min to rest. Carve the beef into thick slices and serve with a crunchy salad made with the spinach, cucumber, celery, radish and sesame seeds.