To make the Teriyaki sauce, combine all ingredients in a saucepan. Stir the mixture well. Put it on medium heat and bring to a boil. Turn the heat down to low and simmer for a couple minutes. Stop the heat and cool the mixture. It's now ready to use but if you have any left you can store the sauce in a clean bottle in the fridge.
Rub the steaks with the oil, salt, and pepper. Grill the steaks on a hot barbecue for 2 minutes on each side so that they're medium rare. Leave to rest. While the beef is resting rub the asparagus with a little oil then lay on the barbecue grill for 2-3 minutes, turning every now and then until they start to soften but still have 'bite' and have little chars on them.
Split the asparagus between two plates. Carve the steaks into 1 cm strips and lay on top of the Asparagus pouring over the meat juices left from resting. Cover each steak with a few tablespoons of Teriyaki sauce and sprinkle with the Togerashi if using and sesame seeds.