Place the flank steak in the freezer for 15 minutes to firm up, then slice it very thinly against the grain into bite-sized strips.
Season the beef strips evenly with the sea salt and black pepper.
In a small bowl, whisk together the tamari, minced fresh ginger, and minced garlic to create the stir-fry sauce.
Heat a large wok or cast-iron skillet over high heat and add the sesame oil, swirling to coat the surface.
Add the beef strips in a single layer and sear for 1-2 minutes until browned and caramelized, then remove the beef from the pan and set aside.
In the same pan, add the broccoli florets and sliced bell peppers with a splash of water, covering for 2 minutes to steam-fry until tender-crisp.
Return the beef to the pan and pour the tamari sauce over the mixture, tossing everything together for 1 minute until the sauce thickens and coats the ingredients.
In a separate small skillet, lightly sauté the cauliflower rice for 3-4 minutes until warmed through and fluffy.
Serve the beef and vegetable stir-fry over the cauliflower rice and garnish with a sprinkle of sesame seeds.