Whisk together hoisin (or gochujang), sambal oelek (or sriracha), red pepper flakes (if using), liquid aminos, honey (or maple syrup), rice vinegar, sesame oil, and minced garlic in a bowl or small blender until smooth; set the sauce aside.
In a mixing bowl whisk the egg, then add the thinly sliced flank steak and toss to fully coat each piece.
Add the gluten-free flour and sea salt to the bowl and stir until the beef pieces are evenly coated; pieces may clump together but will separate during cooking.
Heat a large skillet over high heat and add the avocado oil. When the pan is very hot, add the coated beef in a single layer and let it brown, stirring occasionally, for about 3–5 minutes until golden and crisp.
Add the chopped onion and red bell pepper to the skillet and stir-fry with the beef for 3–4 minutes, until the vegetables begin to soften.
Pour the prepared sauce into the skillet and stir to coat the beef and vegetables evenly.
Continue cooking, stirring occasionally, until the sauce thickens and becomes sticky, about 5–8 minutes; remove from heat and serve.
