Heat a 10-inch skillet over medium-high heat.
Add the ground beef and chopped onion to the hot skillet and cook, stirring and breaking the meat into crumbles, until the beef is browned and the onion is tender, about 7–10 minutes.
Spoon off or drain excess fat from the skillet.
Reduce the heat to low and stir in the canned cheddar cheese soup, ketchup, and black pepper until evenly combined.
Simmer over low heat, stirring occasionally, until heated through, about 3–5 minutes.
Toast the split hamburger buns and spoon the meat mixture onto the buns to serve.