Slice the flank steak against the grain into very thin strips and season evenly with sea salt and black pepper.
In a small glass jar or bowl, whisk together the coconut aminos, minced ginger, minced garlic, and arrowroot powder until the starch is fully dissolved.
Heat a large cast-iron skillet or wok over medium-high heat and add the sesame oil, swirling to coat the surface.
Working in batches if necessary, add the beef to the hot pan in a single layer and sear without moving for 90 seconds to develop a deep brown crust, then flip and cook for 1 more minute.
Remove the beef from the pan and set aside; immediately add the broccoli florets and red bell pepper with two tablespoons of water.
Cover the pan with a lid for 2 minutes to allow the vegetables to steam until they are vibrant green and tender-crisp.
Remove the lid, return the beef to the pan, and pour the sauce over the mixture, tossing everything together for 1 minute until the glaze is thick and glossy.
Transfer to a plate and garnish with toasted sesame seeds for a final layer of texture.