Step 1: Sauté Aromatics - Heat the avocado oil in a deep-frying pan or wok over medium heat. Add the onion and mushrooms and cook for 5 minutes, until they are soft and lightly golden.

Step 2: Combine Chicken & Sauce - Add the shredded chicken to the pan and cook for 3-4 minutes while stirring. In a separate small bowl, combine the soy sauce, rice vinegar, brown sugar, and chicken stock. Stir to mix well. Pour 1/3 cup of this soy mixture into the pan with the chicken and vegetables. Stir to combine and bring to a simmer.

Step 3: Cook the Omelet - Pour the lightly beaten eggs evenly over the simmering chicken and vegetable mixture. Do not stir. Cover the pan and cook for about 2 minutes, or until the eggs are just lightly set and the yolks are slightly runny in the center (adjust cook time based on your preference).

Step 4: Assemble & Serve - Add the rice and the remaining soy mixture to a deep serving bowl. Carefully cut the chicken and egg mixture into large pieces (about 2-4 pieces) and place them on top of the rice. Top with the shaved celery and sprinkle with sesame seeds. Drizzle with Tonkatsu sauce, if desired, and serve immediately.