Before cooking the lentils, sort through them to remove any stones or debris. Bring a large sauce pot of water to a boil over high heat, then add the lentils. Reduce the heat to low, cover, and let simmer for 20 minutes*. Test the lentils to make sure they are tender and then drain in a colander. Set the cooked lentils aside until you are ready to add them to the sloppy joes. The lentils can be simmering while you make the rest of the sloppy joes.
Mince the garlic and finely dice the onion. Add both to a large skillet along with the olive oil and sauté over medium heat until they begin to soften (3-5 minutes).
While the onion and garlic are cooking, finely dice the bell pepper. Add the diced bell pepper to the skillet and sauté a few minutes more.
Add the ground beef and continue to cook until the beef is fully browned (about 5-7 minutes).
Add the tomato sauce, tomato paste, cider vinegar, brown sugar, dijon, chili powder, and salt. Stir well and let simmer for 5-10 minutes, or until the sauce has thickened to your liking.
Stir in the cooked lentils and heat through. Taste and adjust the salt, if needed. Serve warm on a bun!
*Brown lentils take approximately 20 minutes to cook. If you are using a different variety (like green or French lentils), be sure to check the packaging for recommended cooking time.