In a food processor or blender, add the onion and garlic with about 1/4 cup of water. Pulse until pureed.
I prefer to marinate the beef in a gallon-size plastic storage bag, but a bowl or a flat storage container will suffice.
Tip: If you're using a bag, place the bag in a tall container to make mixing the marinade easier and without having to add another dirty dish to the pile in the sink.
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After you've selected your marinating vessel, mix together the onion/garlic puree, soy sauce, 1/4 cup of water, brown sugar, and sake. Add the beef and make sure every every inch is coated. Marinate for at least 2 hours, and overnight if you have time.
Heat the grill on high. Grill for about 5-7 minutes on each side for medium doneness.
Serve and eat right away!