Prepare the Steak: Slice the sirloin steak into ¼-inch slices. Lay the slices flat, cover with plastic wrap, and use a meat pounder to flatten the meat. Place the flattened steak slices into a Ziploc bag, add the cornstarch, and massage the bag to ensure each piece is fully coated.
Cook the Steak: Pour the canola oil into a large frying pan and heat over medium-high heat. Add the steak slices to the pan in a single layer and cook for 30 seconds per side (1 minute total). Cook the steak in batches to avoid overcrowding the pan. Remove each batch of steak as it finishes cooking, set aside on a plate, and sprinkle with salt.
Make the Sauce: In the same pan, add the grated ginger, minced garlic, and red pepper flakes or chili oil (if using). Sauté for 10-15 seconds until fragrant. Stir in the tamari (or soy sauce), water, and brown sugar, bringing the mixture to a boil.
Combine and Finish: Return the cooked steak to the pan, folding it into the sauce. Let the sauce thicken for 20-30 seconds. The cornstarch from the steak should help thicken the sauce. Turn off the heat, add the scallions, and stir to combine.
Serve: Serve the Mongolian beef over white rice, rice noodles, or in lettuce wraps.