Preheat oven to 400°F. Then spray a 2-quart baking-dish with olive oil.
Heat the olive oil in a large-skillet over medium-high-heat for2 minutes. Then add onions and beef.
Cook, stirring to break up the beef, till the beef is browned and the onion is tender for 5 minutes. When there are fluids in the bottom of the skillet, then drain the mix and then return it to the skillet.
Stir in the black pepper, garlic and kosher salt. Cook, stirring for a few minute.
Turn off the heat. Stir in the unsweetened ketchup, mayonnaise, mustard and 1 cup cheddar.
Transfer the mixture to the prepared baking-dish. Sprinkle with the remaining cheese.
Bake until the cheese is melted and the casserole is heated through, for 15 minutes.
Or, if your baking-dish is broiler-safe, broil the casserole 6-inches below the heating element (not straight under ) till cheese is melted, for 1-2 minutes.