Heat the oil in a large skillet over medium high heat. Add the onion, celery, and carrot. Cook until the onion is tender. About 5 minutes.
Turn heat down to medium and add in the ground beef and garlic. Cook, while breaking up the meat with a large spoon. Season with salt and pepper.
Continue to cook until the meat is cooked through.
Add the tomato sauce, water, honey, chili powder, vinegar, mustard, and lentils to the pan.
Bring moisture to a boil and then turn heat down to low. Cover, and simmer until lentils are cooked through (usually about 20-25 minutes.) Add more water during this time if mixture becomes too dry for the lentils to cook. Check every couple of minutes past 20 minutes to not overcook.
Serve warm in lettuce leaves.
This variation of a sloppy joe was created after eating traditional sloppy joes and feeling like I was eating way too much meat and not fueling myself with nutrients. So then, I tried making it with lentils instead. And that was sloppier than a sloppy joe! It seemed like the perfect ratio to have a little ground beef with lentils. We LOVE this combo!