Heat oil in a large sauce pan over medium heat. Add onion and green pepper and cook, stirring occasionally, until starting to brown, about 5 minutes. Add lentils, water, and oregano and bring to a boil. Reduce heat to low, cover and cook, stirring occasionally, until lentils are tender, about 45 minutes-1 hour.

Stir in remaining ingredients and taste. Season with additional salt and some black pepper if desired. Serve on toasted buns with pickle relish and mustard.