Bring lentils and 2 cups water to a boil in a small saucepan, reduce heat and simmer for 25 to 30 minutes until tender. Drain any excess water.
Brown beef, onion and bell pepper in a skillet over medium-high heat for 8 to 10 minutes, breaking up the meat and cooking it to 160°F. Drain excess fat.
Stir in cooked lentils, tomato sauce, ketchup, brown sugar, mustard, vinegar, Worcestershire, salt and pepper. Simmer for 10 minutes until the filling forms a soft mound on a spoon.
Freeze after cooking: divide filling among 6 shallow containers, refrigerate within 2 hours and chill fully. Seal, label and freeze for up to 2 months, keeping buns separate.
Thaw filling overnight in the refrigerator. Reheat in a covered skillet over medium-low heat for 8 to 12 minutes, adding reserved water as needed, until it reaches 165°F.
Toast buns lightly in a dry skillet and fill each with one portion of the hot beef-lentil mixture.