Coat a medium heavy skillet with cooking spray and place over medium-high heat. Crumble in the beef and cook, stirring often until the beef is broken up and mostly browned, 3 to 5 minutes. Add the onion, cumin, chili powder, garlic powder and salt and cook, stirring until the beef is no longer pink and the spices are fragrant, 1 to 3 minutes longer. Remove from the heat and sprinkle the flour over the meat mixture. Stir to combine.
In the insert of a slow cooker whisk together the tomato puree, brown sugar, and vinegar. Stir in the beef mixture, lentils and chilies. Cover the slow cooker, and cook for two hours on the high setting. Serve the saucy meat on buns topped with cheese.