Pour 1 cup of boiling water over the dried shiitake mushrooms. Let them rehydrate for at least 30 mins. Once they’ve softened remove their woody stems and discard. Thinly slice and set aside. Retain the soaking liquid.
Bruise the 3 inch piece of ginger with the flat of your knife. Add to a pressure cooker or pot along with rolled oats, spring onions (the bottom parts), salt and the stock cube. Pour in 3 cups of water and the shiitake soaking liquid.
Clamp on the lid and cook until the oats have completely broken down. This takes about 7 mins in a pressure cooker; will take longer in a pot. (Keep an eye out in the pot and add more water if necessary.) You want the consistency of a thick porridge.
Meanwhile sauté the mushrooms: add oil to a large, wide pan over high heat and first toss in the shiitakes. Cook, moving them around for about 3 mins.
Tip in the button mushrooms and continue cooking over a high heat until they brown and lightly caramelize. Let all the water evaporate before you take them off the heat. Toss with toasted sesame oil if using.
Fish out the piece of bruised ginger from the oats and discard. Divide between bowls and top with mushrooms, ginger slices, chilli oil, 1 teaspoon of soy in each bowl, and spring onion tops. Serve hot.