Boil lentils and 2 cups water in small saucepan. Reduce heat; simmer 25 to 30 minutes until tender. Drain excess water.
Brown beef, onion, and bell pepper in skillet over medium-high 8 to 10 minutes, breaking up meat, to 160°F. Drain excess fat.
Add cooked lentils, tomato sauce, ketchup, sugar, mustard, vinegar, Worcestershire, salt, and pepper. Simmer 10 minutes until filling softly mounds on spoon.
Divide cooked filling among 6 shallow containers; refrigerate within 2 hours and chill fully. Seal, label, and freeze up to 2 months; keep buns separate.
Thaw overnight in refrigerator. Reheat covered in skillet over medium-low 8 to 12 minutes to 165°F, adding reserved water as needed.
Lightly toast buns in dry skillet; fill each with one portion hot filling.