To make the filling, combine the ground chicken, water chestnuts, rice wine, oyster sauce, and cornstarch in a bowl until well mixed.
To make the dumplings, place a heaping teaspoon of filling in the center of a wonton wrapper. Cover the remaining wrappers with a damp kitchen towel to prevent them from drying out. Bring the sides of the wrapper together, bunching them around the filling and smoothing any pleats. Flatten the bottom of the dumpling by tapping it against a flat surface, and squeeze the sides of the dumpling gently so the filling plumps out of the top. Cover the formed dumplings with a damp kitchen towel to prevent them from drying out.
To make the sauce, heat a wok or stir-fry pan over high heat until hot. Add the oil, swirling to coat the sides. Add the garlic and ginger and cook, stirring until fragrant, for about 10 seconds. Add the tomatoes and stir-fry until the excess liquid is evaporated, for 3 to 4 minutes. Add the broth, chilli garlic sauce, soy sauce, and sesame oil and bring to a boil. Add the cornstarch mixture and cook, stirring until the sauce boils and thickens for about 1 minute. Keep the sauce warm over low heat.
Prepare a wok or stir-fry pan for steaming. Line a steaming basket with the lettuce leaves. Arrange the dumplings, without touching one another, in the prepared basket. Cover and steam over high heat until the filling is cooked through, for 4 to 5 minutes.
Pour the sauce onto a serving plate. Arrange the siu mai on top of the sauce. Sprinkle the cilantro and green onion on top and serve.