This is a good way to stretch ground beef if you do not have enough to go around---but this is still a really, really good chili and I think it tastes better than just beef chili.
Brown 1 pound (or less, if your family is smaller and you do not want leftovers) ground beef with a chopped up onion. Diced garlic is good also. Drain this, then add a can or two of chopped tomatoes, a packet of chili seasoning, and enough water to make it the consistency that you like. Let this simmer while you cook the lentils. Put a cup of dried brown lentils in a pot, cover with plenty of water, and bring to the boil. Boil them until tender (about 30 minutes---there is no need to presoak the lentils). When they are done, drain them and add them to the chili. Let this simmer for a bit to blend the flavors.
Serve in bowls. You can top with any of the following---oyster crackers, shredded cheddar, sour cream. It tastes even better if you refrigerate overnight and serve the next day.