Pat the beef steak dry with paper towels and slice thinly against the grain. Toss with baking soda and marinate in the refrigerator for 30 minutes.

Whisk soy sauce, brown sugar, and cornstarch in a medium bowl until smooth, then set aside.

Heat oil in a large wok over medium-high heat and stir-fry bell peppers for 3-4 minutes until tender-crisp.

Increase heat to high, add marinated beef, and cook for 2-3 minutes until browned.

Pour prepared sauce over beef and vegetables, stirring quickly to coat and thicken for 1-2 minutes.

Transfer to a serving platter, garnish with sesame seeds or green onions, and serve immediately.