Select the 'saute' function of your Instant Pot.
To the stainless steel insert, add olive oil and onion. Saute for 5-7 minutes, until onion is soft and translucent.
Make a space in the middle of the pot and add the garlic; saute for 1 minute.
Add the tomato paste, and saute, stirring constantly, for 2 minutes.
Add 1 cup of the stock to the pot. Use a spatula or wooden spoon to scrape all the bits off the bottom of the pot.
Add the remainder of the stock, as well as carrots, celery, lentils, thyme, rosemary, bay leaf and salt. Stir, and put the lid on the Instant Pot.
Set the steam release handle to the 'sealing' position, then pressure cook on high pressure for 15 minutes.
When the cooking cycle has completed, allow the Instant Pot to naturally depressurize for 10 minutes- do not touch.
Carefully set the steam release handle to the 'venting' position to release the remaining pressure.
Add the lemon juice, then using an immersion blender, pulse the soup until slightly thickened.