Place steak in freezer until firm, about 20 minutes. Thinly slice across the grain. Meanwhile, in small bowl, whisk together oyster sauce, chili garlic sauce, pinch each salt and pepper and 1/3 cup water. Set aside.

In bowl, toss together beef, garlic, egg yolk, cornstarch; and pinch each salt and pepper. (Make-ahead: Cover and refrigerate for 1 hour.)

In large nonstick skillet or wok, heat 1 tsp vegetable oil over medium-high heat; stir-fry half of the beef mixture until browned, about 3 minutes. Transfer to plate. Repeat with oil and remaining beef mixture. In same pan, heat 1 tsp vegetable oil over medium-high heat; stir-fry onion and shiitake mushrooms until onion is golden and mushrooms are softened, about 5 minutes. Add broccoli florets, red pepper and 2 tbsp water. Stir-fry until broccoli is tender-crisp, about 8 minutes. Return beef and any accumulated juices to pan. Stir in oyster sauce mixture; cook, stirring, until slightly thickened, about 1 minute.