Mix sauce ingredients in a small bowl. Toss chicken with 1 ½ tbsp of the sauce, set the rest aside for drizzling.
Heat 1 ½ tbsp oil in a large heavy based pot on high heat. Cook half the mushrooms with a pinch of salt and pepper until some of the mushrooms have a light tinge of golden on the surface, but don't try to cook them all the way through. Remove into bowl. Repeat with remaining mushrooms.
Heat the remaining 1 tablespoon of oil until hot. Add the chicken and stir for 1 minute, just until the surface is sealed but inside still raw. Add white part of green onion, garlic and ginger, stir 30 seconds. Add rice, stir briefly just to coat in oil.
Add stock and ginger slices. Bring to a rapid simmer. Scatter mushrooms across surface, then let it come to a rapid simmer again.
Put the lid on, reduce stove to low (or medium low, for weak burners). Cook for 20 minutes (do not stir) until the liquid is fully absorbed.
Remove pot from the stove with the lid still on and rest for 10 minutes.
Drizzle remaining sauce all over, add green part of green onions. Gently toss until the green onion is mostly wilted. Serve!
