Heat the olive oil in a large frying pan and add the chopped onions and garlic. Cook them over medium heat until the onions are soft. Make sure to keep stirring occasionally so that the crushed garlic clove doesn't burn.
While the onions are cooking, wash and finely chop the mushrooms and place to one side.
Run your fingers lightly along the stems of the thyme to take off all of the tiny leaves (you don't need the stems) and set aside.
Once the onions and garlic are soft and almost clear to look at, add in the mushrooms and cook for around 8 minutes or until the water from them has cooked off.
Then add in the the soy sauce and thyme leaves and give it all a good stir.
Reduce the heat until most of the liquid has cooked off and remove the pan from heat.
Finely chop your chestnuts and add to the pan, along with the nutritional yeast and give it another good stir.
Give it a taste and add the salt and pepper to your liking.