Add the ground beef and diced onion to a large skillet and cook and crumble on the stove-top until the ground beef is browned and the onions are translucent. Drain off any excess fat and add the cooked ground beef and onions to a 4 to 5 quart slow cooker.
Add the remaining ingredient to the slow cooker and gently stir to just combine the ingredients a little.
Cover and cook on LOW for 2 to 3 hours, stirring every 30 minutes to ensure the cheese is melting evenly.
Turn slow cooker to the WARM setting to keep the dip warm for serving with tortilla chips or crackers.
This recipe is best cooked on the LOW setting as the HIGH setting is more prone to burn the cheese.