For the garlic chips:
1. First make the garlic chips.
Heat the oil in a small but deep frying pan until screaming hot, add in the thinly sliced garlic. The garlic should sizzle. Carefully fry them, adjusting the temperature to a medium -low heat so they brown but do not burn. This should take 5-6 minutes. Once the garlic begins to color, remove them and drain on kitchen paper. Be careful not to cook them past the point where a golden color has been achieved as they will still continue cooking after draining.
For the onion garnish:
2. Next prepare your onion salad garnish.
Soak the thinly sliced onion covered in salted water for about 10 minutes. Then rinse well and squeeze the water out completely. If you don't have time to soak, you can sprinkle the salt over the sliced onion and rub them well with your hands to incorporate the salt until the onions are soft and the liquid has been extracted from them. Then rinse them well and squeeze out the water.
For the beef:
3. Now prepare your beef.
4. Combine your mirin and soy sauce in a shallow dish, large enough to dip your meat in once it is cooked, and set aside.
Heat a frying pan until hot. Brush the beef with the vegetable oil and cook in the hot pan until browned all over; if the beef is too thick, cut the beef lengthways to a thinner chunk. It is important to sear all sides of the beef properly first. Once all sides are seared, reduce your temperature and slowly cook the beef for 8-10 more minutes until desired level. This is best served medium rare which should only take about 3-4 minutes per side depending on thickness of your cut.
5. Once the beef is cooked to the level you wish, remove the beef from the pan and quickly transfer the beef and the pan juices into the mixture of soy sauce and mirin. Leave your beef in the marinade for 20-30 minutes to marinate at room temperature, this will make it easier to slice and more tender.
6. Drain the soy and mirin marinade in to a separate bowl. This reserved marinade will be re-used to make the sesame sauce so be sure all the juice from the pan go in to the soy sauce mixture. as this way the sauce will retain the beef flavor. Now you can make your sesame sauce.
For the sesame sauce:
1. To make the sesame sauce, take your mixture of soy sauce and mirin (which cooked beef was dipped in) and add in tahini paste, water, and sugar. Mix well. You may add a teaspoon of rice vinegar if you wish. The consistency should be thick and resemble evaporated milk.
2. Slice the beef into about 5mm/1/4 inch thin slices.
3. Place a bed of the onion salad on a serving plate and and place your beef slices on top. Drizzle over with the sesame sauce and add more on the side for dipping. Sprinkle the garlic chips over top.