Place the ground beef, onion, and garlic in a skillet over medium heat. Cook until the beef is browned and the onion softens, then drain any excess fat.
Transfer the beef mixture to the slow cooker. Add the soy sauce, fish sauce, brown sugar, and red pepper flakes. Stir everything together until well combined.
Cover and cook on low for 4 to 5 hours.
Turn off the heat and stir in the torn Thai basil leaves until they wilt slightly.
Spoon the beef over bowls of cooked jasmine rice and serve right away.