In a mixing bowl or large measuring cup, whisk together the marinade: 3 tbsp liquid aminos, 1 Tbsp rice vinegar, 1 tsp sambal oelek, 1 tsp sesame oil, 1 Tbsp fresh grated ginger, 1 tsp garlic powder, and 2 Tbsp tapioca flour.

Slice 2 lbs sirloin steak into thin ¼-inch strips about 2 inches long. Place the steak in a resealable plastic bag or a bowl, pour in the marinade, press out excess air or cover, and turn to coat. Refrigerate and marinate at least 1 hour and up to 24 hours.

While the steak is marinating, whisk together the sauce in a separate bowl or measuring cup: 2 Tbsp oyster sauce, 1/4 cup liquid aminos, 1 Tbsp gochugang, 1/4 cup water, 2 Tbsp coconut sugar or pure maple syrup, 2 Tbsp rice vinegar, 1 Tbsp sambal oelek, 1 tsp sesame oil, 1 tsp szechuan peppercorn, and 3/4 tsp Chinese 5 spice. Set the sauce aside.

When ready to cook, heat a large heavy skillet (nonstick or well-seasoned) over high heat until very hot. Remove the steak from the marinade, shaking off excess (discard any used marinade). Add the steak to the skillet in a single layer — work in batches if needed to avoid overcrowding — and cook, stirring occasionally, until the strips develop a deep golden-brown crust, about 2–4 minutes per batch. Transfer cooked steak to a plate and keep warm.

Add the sliced 1 medium yellow onion and 1 red bell pepper (cut into match sticks) to the same hot skillet. Sauté, stirring frequently, until the onion is translucent and the pepper is tender-crisp, about 3–5 minutes. If the pan is excessively dry and vegetables begin to burn, add a tablespoon of the prepared sauce to deglaze.

Return the cooked steak to the skillet with the vegetables, pour in the prepared sauce, and stir to combine. Bring to a simmer and cook, stirring constantly, until the sauce thickens and coats the beef and vegetables, about 3–5 minutes. Ensure the beef is cooked through.

Remove from heat and serve immediately.