Place the lentils in a pot with enough water to cover them by about 1.5 inches. Turn the heat to high, and bring to a boil, then reduce heat to medium low. Place a lid on the pot and tilt it to vent out the steam.
Cook until lentils are tender, about 25-30 minutes. Drain out any excess water from the pan.
In a large pan, cook the onion in the olive oil over medium heat until tender, about 7 minutes.
Add the lentils to the onions, and stir. Add the remaining ingredients, and heat over medium low heat for about 15 minutes so the flavors can combine.
Add more salt and pepper as desired.
If desired, toast the buns. If you need a gluten free bun, you could check Schar brand, which is gluten free, but does contain soy.
Top toasted buns with sloppy joe mixture and serve. Garnish with chopped onion if desired.