Beef Stir-Fry with Bell Peppers and Black Pepper Sauce

Combine 1 tablespoon water and baking soda in medium bowl. Add beef and toss to coat. Let sit at room temperature for 5 minutes. Whisk 1 tablespoon soy sauce, 1 tablespoon sherry, 1½ teaspoons cornstarch, and ½ teaspoon sugar together in small bowl. Add soy sauce mixture to beef, stir to coat, and let sit at room temperature for 15 to 30 minutes. Whisk remaining ¼ cup water, remaining 2 tablespoons soy sauce, remaining 2 tablespoons sherry, remaining 1½ teaspoons cornstarch, remaining 2 teaspoons sugar, oyster sauce, vinegar, sesame oil, and pepper together in second bowl. Heat 2 teaspoons vegetable oil in 12-inch nonstick skillet over high heat until just smoking. Add half of beef in single layer. Cook without stirring for 1 minute. Continue to cook, stirring occasionally, until spotty brown on both sides, about 1 minute longer. Transfer to bowl. Repeat with remaining beef and 2 teaspoons vegetable oil. Return skillet to high heat, add 2 teaspoons vegetable oil, and heat until beginning to smoke. Add bell peppers and scallion greens and cook, stirring occasionally, until vegetables are spotty brown and crisp-tender, about 4 minutes. Transfer vegetables to bowl with beef. Return now-empty skillet to medium-high heat and add remaining 4 teaspoons vegetable oil, scallion whites, garlic, and ginger. Cook, stirring frequently, until lightly browned, about 2 minutes. Return beef and vegetables to skillet and stir to combine. Whisk sauce to recombine. Add to skillet and cook, stirring constantly, until sauce has thickened, about 30 seconds. Serve immediately.