Cut the meat into 2-inch pieces, then thinly slice each piece against the grain. In a medium bowl, combine the beef, 1 tablespoon of the soy sauce and the pepper. Stir to coat evenly. Set aside.

Meanwhile, break open about half of the chilies, then transfer all of them to a 12-inch nonstick skillet. Toast over medium, pressing the pods against the skillet, until fragrant and darkened in spots, 1 to 3 minutes. Transfer to a large bowl and add the cumin seeds. In a small bowl, stir together the garlic and 2 teaspoons of the grapeseed oil to form a paste.

Add 2 teaspoons of the remaining grapeseed oil to the empty skillet and heat over high until shimmering. Add the onion and cook, stirring occasionally, until charred in spots and partially softened, 3 to 5 minutes. Transfer to the bowl with the cumin seeds and chilies. Return the skillet to high heat. Add 1 teaspoon of grapeseed oil, then arrange half of the beef in a single layer in the skillet and cook, undisturbed, until deeply browned on the bottom, 1 to 2 minutes. Stir, scraping the bottom of the pan, and cook until no pink remains, 60 to 90 seconds. Transfer

to the bowl with the onion. Repeat with the remaining grapeseed oil and beef.

In the skillet over low, cook the garlic paste, mashing the mixture, until aromatic, about 1 minute. Add the beef mixture. Stir in the remaining 2½ tablespoons of soy sauce, the vinegar and sugar. Increase the heat to medium­-high and cook, stirring constantly and scraping up any browned bits, until the liquid is thickened, about 1 minute. Off heat, stir in the sesame oil and cilantro.