1 C. soy sauce
1 C. washed raw cane sugar or 1/2 C. granulated sugar plus 1/2 C. firmly packed brown sugar
3/4 C. thinly sliced onion
1/2 C. sake
1/2 C. mirin (sweet sake; or 1/2 C. additional regular sake plus 1 tsp. sugar)
1 T. minced garlic
1 T. thinly sliced fresh ginger
1/2 tsp. coarse-ground pepper
1/2 tsp. dry mustard
1 beef tri-tip (1 1/2 to 2 lb.), fat trimmed, rinsed, and patted dry

Stand a gallon-size zip-lock plastic bag in a bowl. Pour in soy sauce, sugar, onion, sake, mirin, garlic, ginger, pepper, and mustard. Hold bag closed and shake until sugar is dissolved. Add beef; seal bag. Chill at least two hours or up to 1 day, turning occasionally. Lay tri-tip on a lightly oiled barbecue grill over a solid bed of medium coals or medium heat on a gas grill (you can hold your hand at grill level only 4 to 5 seconds); close lid on gas grill (discard marinade). Cook, turning every 5 minutes, until rare in center of thickest part (cut to test; 125° to 130° on a thermometer), about 25 minutes, or until as done as you like. Transfer tri-tip to a cutting board. Let rest about 5 minutes, then cut across the grain into thin, slanting slices.