Add the oil to your frying pan on a low to medium heat, then add in the chopped onions.
Fry for 8-10 minutes until soft.
Next, add the garlic and fry for a minute, then add in the squash, fry for 2-3 minutes.
Now add in the pearl barley. Stir to combine thoroughly.
Add a ladle full of stock at a time until all the liquid is absorbed, and the squash is soft and melting into the risotto.
Now add in the mushrooms, nutritional yeast, lemon juice and thyme. Stir to combine and heat until the mushrooms have softened.
Drizzle in the truffle oil, season with salt and pepper.
Turn off the heat and cover the pan with a lid while you make the mushrooms.