Set slow cooker to High. Add ground beef and break it into smaller chunks.
In a measuring cup, combine warm water, dried onion, brown sugar, beef base, chicken base, apple cider vinegar, Worcestershire, and soy sauce. Stir until dissolved.
Pour seasoning liquid over the beef and stir until the meat is coated.
Cover and cook on High for 1 hour.
Remove lid and break the meat into very small crumbles with a meat chopper or firm spatula.
Turn slow cooker to Low and cook uncovered for 2 1/2 to 3 hours, stirring occasionally and breaking up any clumps.
Continue cooking until most liquid has evaporated and the beef is finely crumbled with a glossy sheen.
Serve a generous spoonful on warm hamburger buns with ketchup, mustard, pickles, or other toppings.
Notes
Use lean ground beef, ideally 90/10 or 93/7, so the meat stays juicy without getting greasy. Do not skip the uncovered cook-down because that step concentrates the flavor and creates the loose meat texture. Leftover meat keeps refrigerated for up to 4 days or frozen for up to 3 months. Reheat with a splash of broth or water.