Start by slicing the tops off of the peppers. Using a sharp knife, remove the ribs and seeds from the inside.
In a large bowl mix 2 cups cooked brown rice, 1 cup cooked black lentils, 1 cup shredded cheddar cheese, ¼ cup tomato paste, the diced onion, and diced celery.
Mix well, and season with salt and pepper.
Place the trivet into the Instant Pot insert, and add one cup of cold water.
Place the peppers into the Instant Pot, resting on the trivet.
Remove one pepper from the Instant Pot, and fill with the rice stuffing. Really pack it in there.
Once filled, return the pepper to the Instant Pot, and repeat with remaining peppers.
Place the lid on the Instant Pot, and seal. Ensure the valve is set to "sealing".
Set the Instant Pot to Manual / High Pressure for 10 minutes.
Once the cooking cycle has completed, flip the vent to "venting" and quick release the pressure.
Remove the lid, and sprinkle the peppers with the remaining half cup of cheese. Replace the lid onto the Instant Pot for 2-3 minutes to allow the cheese to melt.
Carefully remove the peppers from the Instant Pot using tongs, or lift the trivet out carefully.
Transfer to a dish, and serve immediately.