Wrap beef tightly in plastic wrap and place on plate or small baking sheet. Freeze for 15 to 30 minutes, until partially frozen and firm to the touch.
Unwrap meat. With sharp chef’s knife, slice meat against the grain into strips that are 2 in (5 cm) long, 1 in (2.5 cm) wide and 1⁄8 in (3 mm) thick. The easiest and most efficient way to do this is to portion beef into pieces that are 2 in (5 wide and 1 in (2.5 cm) thick and then cut those pieces crosswise into strips that are 1⁄8 in (3 mm) thick.
Put meat strips in large bowl, drizzle oil over them and then sprinkle with 2 tbsp of the yaji. Carefully toss and massage yaji into meat until it is evenly coated. Season with another 2 tbsp of the yaji.
Working with one piece at a time, thread beef onto metal or wooden skewers, piercing each piece in two places to secure it, then spread meat out on skewer so it will cook evenly. Leave 2 in (5 cm) of skewer exposed at either end.
Place completed skewer on platter and repeat with the remaining beef. Once all skewers are assembled, sprinkle the remaining yaji on both sides. Cover and refrigerate for at least 30 minutes or up to 8 hours before grilling.
If using charcoal grill, light chimney starter full of charcoal. When coals are hot and covered with gray ash, carefully spread onto one side of grill. If using gas grill, set burners on one side of grill to high. (Leave the other side off.) Cover and preheat for 5 to 10 minutes. Clean and oil the grilling grate.
Place skewers on grill over direct heat and cook for about 8 minutes, turning them over once, until charred on both sides and cooked through. Transfer to serving platter and let rest for 2 to 3 minutes before serving. (Alternatively, let skewers rest for 1 to 2 hours, then reheat as needed.)
Serve the skewers with the Yajin Kuli, onions and tomatoes.