Fry the onion on the oil for around 5 minutes over a medium heat until they start to become soft but do not burn them.
Add the mushrooms and fry for a further 3 minutes.
Mix together the garlic, chilli flakes, agave nectar, vinegar, mirin, soy sauce and sesame oil and add to the pan and cook for 5 minutes.
Add the vegetable stock and cornflour slurry and stir cooking for a further 2 minutes.
Check the taste and add salt and pepper to taste... if you would like it sweeter you can add a little more mirin or agave nectar.
Add the basil, sesame seeds and spring onion.