Add the onions, garlic and oil to the Instant Pot. Use the sauté button to cook until the onions are nearly translucent.
Add carrots and celery and cook a few more minutes.
Stir in vegetable stock, lentils, and diced tomatoes with their juice. Seal the lid and set the cook time for 15 minutes.
When the cook time is done, do a quick pressure release. Add salt and pepper to taste. Serve with a splash of balsamic vinegar.