Place beef in a bowl with soy sauce, garlic and brown sugar and mix to combine then set aside.
To make the sauce mix all ingredients together in a bowl and set aside.
Heat oil in a large pot over high heat then add beef and cook for about 2-3 minutes.
Remove from pan and set aside.
Add oil to the pan then stir fry red onion and pepper and cook for a minute. Add sauce and pineapple and cook until sauce begins to thicken, this just take about a minute or two.
Next add the cooked beef back to the pot and cook for a minute before removing from the stove. Toss in scallions, then serve.