In a small bowl, whisk together the dressing for the coleslaw: 1/2 cup mayonnaise, 2 Tablespoons sugar, 1 1/2 Tablespoons lemon juice, 1 Tablespoon vinegar, and salt and pepper to taste.
Mix the coleslaw mix and the dressing together until well mixed. Cover with plastic wrap and refrigerate until serving.
In a skillet, combine the ground beef, chopped onion and minced garlic. Cook over medium-low heat, stirring occasionally until meat is no longer pink. Drain off grease.
Add the 2 cups water, tomato sauce, rinsed lentils, ketchup, vinegar, Dijon mustard, and pepper and bring to a boil over medium heat.
Cover and reduce heat to low. Simmer for an hour, or until the lentils are cooked and the mixture is thick.
Serve the sloppy joe meat on buns with coleslaw on the side.