This sauce is borrowed from Roy's restaurant's signature Blackened Ahi with Soy Mustard Sauce and Beurre Blanc, but this sauce is too easy and too good not to parade through all sorts of other foods: brown rice, edamame, fish of all kinds, and, notably, steak. Adapted slightly from —Genius Recipes
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Roy Yamaguchi's Soy-Mustard Sauce (a.k.a. 3-Ingredient Steak Sauce)
Mix the mustard powder and hot water together in a cup to form a paste. Let sit for a few minutes to allow the flavor and heat to develop.
Add the vinegar and soy sauce, mix together, and pass through a fine-mesh sieve into a bowl. Yamaguchi recommends covering and refrigerating for at least 1 hour to allow the flavors to develop, but the sauce will still taste delicious right away if you're in a hurry. Serve with steak, or anything else you like (see headnote above for ideas).
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