Set the instant pot to the "saute" setting. Heat the olive oil, then saute the garlic and onions until fragrant. Add in the celery and carrots and cook for around 4 minutes, stiring often.
Add in the lentils and cook for around 1 minute.
Add in the vegetable broth, diced tomatoes, Italian seasoning, cumin, salt and pepper. Stir to combine.
Cover the Instant Pot and make sure it is in the locked position. Change the setting to manual and set the timer for 14 minutes.
After the soup has cooked for 14 minutes, allow to natural pressure release for 10 minutes.
Remove lid and mix in spinach. Serve immediately.