Turn the pressure cooker on to the saute setting and add the oil or vegetable broth. Saute the onion for two minutes, until it’s just beginning to brown. Add the carrots, celery, garlic, potato, bell pepper, and herbs and saute another minute. Turn off the pot.
Add the tomatoes, broth, and lentils. Cover and set the Instant Pot to manual for 6 minutes with the valve set to sealing. It will take about 15 minutes to pressurize before the pressure cooking begins. Let the pot naturally depressurize for 10 minutes after cooking. Wear an oven mitt and carefully turn the valve to venting. Very hot steam will escape the vent so be sure not to get too close.
When the pot has completely depressurized, take off the top away from your body. Use tongs to remove any fresh herb stems. Stir in the spinach until wilted. Season with salt and pepper to taste.