In a small bowl, mix the soy sauce, rice vinegar, sugar and water.
Mix the cornstarch with 1 tsp of water until no lumps remain, then stir it into the rest of the sauce ingredients.
Heat the oil in a frying pan on medium heat. Add the mushrooms and fry for a few minutes, until they have shrunk down and just started to brown.
Then add the minced garlic and ginger and fry for another minute.
Finally, add the sauce to to the mushrooms in the frying pan and stir constantly as the mixture bubbles up and thickens.
For serving, garnish your teriyaki mushrooms with a sprinkle of sesame seeds and spring onion. Serve with rice or noodles and veggies of your choice.
Notes
If you don't have rice vinegar, then try apple cider vinegar as a substitute. You can use simple white mushrooms for this recipe, or other varieties such as portobello or shiitake. You can also use this method for cooking teriyaki tofu or stir fried veggies. Use tamari sauce instead of soy sauce to make these mushrooms gluten free.