Rinse the barley in a colander under tap water, then leave it for a few minutes to drain before using.
Set the SAUTE mode of your instant pot for 6 minutes.
Add olive oil to the inner pot. Add garlic, celery, coriander stalks, thyme, bay leaf, and onion. Saute them for 20 - 30 seconds to release the aroma of herbs and infuse their flavour into the oil.
Add carrot, zucchini, mushroom, green peas, and pearled barley. Saute for 1 minute.
Add oregano, salt, pepper, stock cube, and water. Stir to combine. Cancel the SAUTE mode. Close the lid of the instant pot.
PRESSURE COOK barley soup for 20 minutes. Allow the steam to release naturally, or you can do a Quick Release manually.
Add fresh greens like cilantro, basil, or celery leaves. Stir to combine. Taste and, if required add more seasoning.
Serve vegetable barley soup warm.