Heat a skillet over medium and add the ground beef, breaking it up as it cooks until no pink remains and brown bits form on the pan.

Add the diced onion and minced garlic to the skillet, stirring until the onion is translucent and fragrant; season with salt and black pepper.

Lay out each slice of bread, divide the warm beef mixture evenly across two slices, and sprinkle each with the shredded cheese.

Top each filled slice with the remaining bread halves to create two sandwiches ready for buttering and toasting.

Spread a thin, even layer of butter on the outside of each sandwich and toast in a skillet over medium-low heat or in a sandwich press until both sides are golden brown and the cheese is fully melted.

Remove the melts from the pan, let them rest for 1 minute to set, then slice and serve immediately.