Brown the ground beef and onions in a large skillet; drain the excess fat, then transfer the browned beef and onions to a large slow cooker or a stockpot. Add the beans,tomatoes,taco seasoning, and ranch dressing mix. Cook in a slow cooker on low for 6 to 8 hours or simmer over low heat for about 1 hour in a pot on the stove. To serve, place a few corn chips in each bowl and ladle soup over them. Top with sour cream.
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