Heat a drizzle of oil in a large pot set over medium-high heat and sauté the onion for 4-5 minutes, until soft. Add the garlic and cook for another minute. Add the beef and cook for about 5 minutes, breaking it up as you cook, until the meat is no longer pink.
Add the tomato sauce (more or less depending on how saucy you like it), ketchup, vinegar, brown sugar, Worcestershire sauce, and salt and pepper to taste. Stir in the dry lentils and 1/4 cup of water, bring to a simmer and cook for 15-20 minutes, or until the lentils are soft and the sauce has thickened. Split the buns or biscuits in half and ladle the sloppy Joe mixture on top. Serves 4.