In a small bowl, whisk together the sauce ingredients until well-combined. Set aside.
In a medium bowl, whisk together the 1 tbsp of water and baking soda. Add the flank steak; set aside for 5 minutes to tenderize.
After letting the beef rest, add the molasses, kosher salt, sugar, and ground pepper to the flank steak; stir to combine. Let marinate in the fridge for 30 minutes–1 hour.
Combine ginger, garlic, Chinese five spice powder, and red pepper flakes in a small bowl.
Heat 12-inch skillet over medium high heat. Add 1 tbsp of oil. Wait for the oil to begin to smoke, and then add the beef mixture. Cook for 3 minutes, tossing with tongs. Remove from pan.
Add 1 tbsp of vegetable oil to now-empty pan. Add the broccoli and stir fry for 30 seconds.
Reduce heat to medium and add 1/3 cup of water to pan. Cover with a lid immediately and let steam for 2 minutes. Lift lid and remove broccoli from skillet.
Return skillet to medium-high heat. Add 1 tbsp of vegetable oil. Wait for the oil to begin to smoke, and then add red bell peppers. Stir fry until blistering, about 2 minutes.
Reduce heat to medium. Form well in middle of the skillet and add ginger-garlic mixture. Cook until fragrant, about 30 seconds, stirring constantly.
Return beef to pan. Give it a quick stir, and then add the broccoli, giving it another quick stir.
Quickly whisk sauce to recombine ingredients, and then add to pan. Stir until ingredients are evenly redistributed; remove skillet from heat.
Optionally garnish with scallions. Serve hot over rice.