Press sauté mode and use the saute function on the instant pot and add the oil.
Once the oil is hot, add the onion, carrot, and celery and sauté for 5 minutes.
Cancel the sauté mode and add the lentil, diced tomatoes, broth, salt, and pepper, stir to mix.
Close the lid of the instant pot and place the valve in the seal position. Select manual high pressure, and add 12 minutes. The pressure will take about 20 minutes to rise and start cooking.
After 12 minutes, release the pressure immediately, and open the lid carefully.