Mix sauce; toss chicken with 1½ tbsp, set remainder aside. Sear mushrooms in 2 batches in a large pot - 1½ tbsp oil with pinch of salt and pepper per batch until lightly golden – but do not cook through! Remove into bowl.
Heat 1 tbsp oil, sear chicken for 1 min. Add green onion whites, grated ginger, garlic, stir 30 sec. Add rice, stir, add stock and ginger slices. Bring to rapid simmer. Top with mushrooms (including any juices accumulated in bowl). Cover and cook on low for 20 min until liquid absorbed (no stirring!).
Rest 10 min covered. Drizzle with remaining sauce, add green part of green onion, toss briefly, and serve.
