Slice the flank steak into 2 inch chunks or into longer, thinner strips across the grain.
In a medium bowl, whisk together: soy sauce, Worcestershire sauce, grated ginger, hot Chinese mustard, red pepper flakes, minced garlic and honey. Whisk to combine and set aside.
Add sesame oil to a medium cast iron pan over medium-high heat.
Once the oil starts to simmer, add the sliced flank steak in one layer. Let it brown nicely for 1-2 minutes without turning it, after that, using tongs to turn beef on the other side to brown for another 2-3 minutes.
Reduce heat to low-medium and add the sauce to the pan. Stir to combine and let the sauce simmer on low heat for about 5 minutes until it reduces in about half and it creates a sticky glaze.
Serve the beef over rice. Garnish with chopped green onions and sesame seeds.