Slice the flank steak against the grain into very thin, bite-sized strips.
In a medium bowl, toss the beef strips with the arrowroot powder, sea salt, and black pepper until every piece is lightly and evenly coated.
In a small glass jar or bowl, whisk together the orange juice, tamari, honey, sesame oil, minced ginger, and minced garlic to create the glaze.
Heat the avocado oil in a large cast-iron skillet or wok over medium-high heat until it is shimmering.
Add the beef to the hot pan in a single layer, working in batches if necessary, and sear for 2-3 minutes per side until the edges are crispy and browned.
Remove the beef from the pan and set it aside on a plate.
Add the broccoli florets and sliced red bell peppers to the same pan with two tablespoons of water; cover with a lid for 2 minutes to steam-soften the vegetables.
Remove the lid, return the beef to the pan, and pour the orange glaze over the mixture.
Toss everything constantly for 1-2 minutes until the sauce bubbles and thickens into a glossy coating.
Transfer to a plate and garnish with thinly sliced green onions and sesame seeds.