Start by preheating the oven to 400°F and greasing a baking dish (I used a 2-quart dish) with butter.
Then, heat the olive oil in a large skillet over medium-high heat. Once hot, add the ground beef and chopped onions. Cook while stirring to break the meat apart, for about 6 minutes.
Drain any excess liquid from the skillet.
Add the salt, garlic, and black pepper. Stir and cook for about a minute.
Turn off the heat. Add the mayo, mustard, ketchup, and 1 cup cheddar cheese.
Transfer the mixture to the baking dish and sprinkle with the remaining cheese. Bake for 12-15 minutes until the cheese melts and the casserole is heated through. You can broil for a few more minutes for the best crust. Enjoy!