Season the ground beef with salt and pepper, then form it into a patty slightly larger than your bread slices. Patties shrink as they cook, so account for this to ensure a good fit.

Preheat your air fryer to 375°F (190°C). This ensures a hot environment for cooking the patty evenly and quickly.

Place the patty in the air fryer basket. Cook for about 10 minutes, flipping halfway through, or until the patty reaches your desired level of doneness. Remove the patty and set it aside.

While the beef patty cooks, caramelize the onions in a skillet over medium heat with a bit of butter or oil until they're golden brown and soft. This may take about 15-20 minutes for the best flavor.

Butter one side of each bread slice. On the unbuttered side, lay a slice of cheese, followed by the cooked beef patty, caramelized onions, another slice of cheese, and finally the second slice of bread, buttered side out.

Carefully place the assembled sandwich back in the air fryer. Cook at 360°F (182°C) for about 3-4 minutes per side, or until the bread is toasted to your liking and the cheese has melted. You may need to press the sandwich down slightly to ensure even cooking.

Once the sandwich is toasted and the cheese melted, remove it from the air fryer. Let it cool for a minute before cutting into halves.