Heat oil in a saucepan and add the onion, cook for 5-8 minutes or until the onions are soft and translucent. Add the garlic and cook for 60 seconds more.
Add the brown rice and water to the pot and stir to combine. Cover with a lid, reduce the heat to low and cook the rice for 30-35 minutes or until tender.
Add the rice and lentil mixture to the peppers. Top each pepper with 1 tablespoon of cheddar. Cover with foil and cook in the oven for 20 minutes or until heated through.