1, Fill a 5-quart saucepan half full with water. Bring to a rolling boil over high heat and add the chicken breast. Remove from the heat and cover tightly. Let stand for 20 minutes. The chicken should be firm to the touch yet still yield a bit. Remove the chicken from the pan. Leave the water in the pan. When the chicken is cool enough to handle, shred it by hand and set aside.
2, Return the water to a boil and add the rice. Parboil for 8 minutes, or until tender but still firm. Drain in a colander and rinse with cold water. Set aside.
3, In the same pan, bring the stock to a boil over high heat. Add the rice, chicken, and mushrooms, lower the heat to a gentle simmer, and cook for 10 minutes, or until the rice expands.
4, Meanwhile, in a skillet, heat the oil over medium heat. Add the onion and cook gently, stirring occasionally, for about 4 minutes, or until fragrant and soft. Add the shrimp and sauté for about 2 minutes, until they curl into corkscrews. Add the crabmeat and stir to distribute. Remove from the heat and set aside.
5, To prevent the tapioca pearls from clumping on contact with the hot soup, put them into a sieve and rinse briefly under cold water. When the rice has expanded in the soup, add the tapioca pearls and cook for another 10 minutes. The opaque pearls will expand and become translucent. At that point, add the shrimp mixture, heat through, and adjust with salt, if necessary.
6, Ladle into individual bowls or a large serving bowl and sprinkle with the scallion and cilantro. Serve immediately.