Slice the flank steak against the grain into thin strips and season with the sea salt and black pepper.
Heat the toasted sesame oil in a large skillet or wok over medium-high heat until shimmering.
Add the beef strips to the skillet in a single layer and sear until browned, about 2-3 minutes, then remove the beef and set it aside on a plate.
In the same skillet, add the sliced yellow onion, red bell pepper, and broccoli florets, sautéing for 4-5 minutes until the vegetables are tender-crisp.
Stir in the minced garlic and grated fresh ginger, cooking for 1 minute until highly fragrant.
Return the cooked beef and any accumulated juices to the skillet and pour in the coconut aminos.
Toss everything together for 1-2 minutes until the sauce has slightly thickened and coated the ingredients.
Serve the stir-fry immediately over the warm cooked brown rice and garnish with sesame seeds.