Place the steak on a cutting board. Using a meat mallet or the bottom of a skillet, pound out the flank steak to help tenderize and thin it out a bit. I like to try and get mine 1/2-inch thick.
Using a very sharp knife, carefully slice the meat against the grain as thinly as possible. For the really long strips of meat, cut them in half lengthwise.
Place sliced steak in a bowl and season with 1 teaspoon salt and 1/2 teaspoon black pepper. Add the 1 teaspoon of oil and the arrowroot. Toss to coat evenly.
Heat a skillet over high heat. Add the remaining 2 tablespoons of oil and when hot, sear the meat on both sides until golden brown, 2 to 3 minutes per side; working in batches as needed. You want each piece to get a good sear for the best results. I do mine in batches until all browned. As they are done, set the browned pieces aside on a clean plate.
In the same skillet, reduce the heat to medium. Add the onions, garlic, bell peppers, and thai chiles and season with remaining 1/2 teaspoon of salt and 1/4 teaspoon of black pepper. Saute until the onions are very tender, 5 to 7 minutes.
Add the browned beef back into the skillet along with the coconut aminos and the fish sauce. Let simmer until the sauce reduces and thickens, 2 to 3 minutes.
Remove from heat and add the basil. Toss the basil into the hot stir fry until just wilted.
Serve immediately with cauliflower rice (optional) and garnish with cilantro (if desired). Serve with a lime wedge.