Scrape out seeds from squash using a spoon. Roast at 350°F on a baking dish, flesh side down, for 20-30 minutes. Let cool, then scrape out strands of flesh using a fork.
In a large skillet, saute onion, garlic, and chicken in oil. Once the chicken is cooked thoroughly, add cabbage, carrots, celery, and soy sauce. As cabbage softens, add squash strands. Toss until well combined. Remove from heat. Squeeze lemon juice over pancit before serving.