Marinate flap meat. Combine meat with egg, soy sauce, oyster sauce, cornstarch, dark soy sauce, and sugar until well combined. Very slowly add water 1 tablespoon at a time, gently squeezing beef to help it absorb the water. Add 1 tablespoon oil, mixing to combine. Let marinate for 1 hour.

Combine ingredients for satay sauce including water, Chinese BBQ sauce, oyster sauce, soy sauce, dark soy sauce, shrimp paste, sugar, curry powder, chili oil, and black pepper in a medium bowl. Set aside.

Wipe pan clean and heat another 2 tablespoons oil over medium heat. Add shallots and garlic and saute until aromatic and softened, about 1 minute. Add satay sauce mixture and bring to a simmer. Lower heat to low and continue to simmer for 5 minutes. Remove 1 cup satay sauce and set aside. Add beef and toss to combine. Bring to a simmer and keep warm.

Combine reserved 1 cup satay sauce with beef broth in a medium pot. Bring to a boil, reduce heat to low and keep warm.

Cook noodles according to directions on package. Drain and portion into 4 bowls. Pour 2 cups soup per bowl and top with satay beef. Garnish with ¼ cup bean sprouts, 1 tablespoon chopped scallions, and 1 tablespoon crushed peanuts per bowl. Serve immediately.