Set the Instant Pot to saute mode and add the olive oil. Once hot, cook the onion, carrots, and celery for 3 minutes until the onion softens.
Stir in the garlic, thyme, and cumin. Cook for 1 minute until fragrant.
Add the lentils, potato, vegetable broth, and diced tomatoes with their juice. Season with salt and pepper.
Cancel saute mode. Secure the lid and set the valve to sealing. Cook on manual high pressure for 15 minutes.
Allow a natural pressure release for 10 minutes, then quick release any remaining pressure.
Open the lid and stir in the spinach until it wilts. Add the lemon juice and adjust salt and pepper if needed.