Set your Instant Pot to the 'Sauté' mode. Once hot, add the olive oil.
Add the diced onion, garlic, carrots, and celery. Sauté for about 5 minutes, or until the vegetables begin to soften.
Stir in the bell pepper and zucchini, and sauté for an additional 2-3 minutes.
Press 'Cancel' on the Instant Pot. Add in the barley, vegetable broth, diced tomatoes, spinach, thyme, oregano, salt, and pepper. Stir well to combine.
Secure the lid on the Instant Pot and make sure the valve is set to 'Sealing.' Set to 'Manual' or 'Pressure Cook' on high pressure for 20 minutes.
Once cooking is complete, allow for a natural release for 5 minutes, then quick release any remaining pressure.
Carefully open the lid and stir the soup. Adjust seasoning if desired.
Serve hot, garnishing with fresh parsley if using.