
Toss the squid with salt and pepper while heating a heavy-bottomed pan over medium to medium-high heat.
Once the pan is hot, add cooking oil to coat the bottom. Add the squid, stir and sauté quickly for up to a minute. No need to cook all the way through yet. Remove the squid from the pan and set aside.
Add a little more oil if needed. Then add sliced onion to the pan and cook for several seconds. Add minced garlic, stir and cook until fragrant.
Add pineapple pieces and give everything a quick stir. Then add tomato and continue to cook for about 1 minute to 1½ minutes. Give the ingredients in the pan a stir from time to time.
Add the squid back to the pan and also add fish sauce. Continue to stir fry constantly for about 30 seconds or until the squid is cooked through. Taste and add more salt or fish sauce to your liking.
Stir in chopped scallion and leaf celery. Sprinkle more black pepper if desired, then transfer to a serving plate and serve immediately.