Preheat the air fryer to 380F. Brush the grate with frying oil and lay out the beef burgers. Fry for 12 minutes on one side.
4 beef beef burger, 2 tbsp oil for frying
Melt the butter in a frying pan and fry the onion over medium heat for 5 minutes, stirring.
1 tbsp butter, 1 onion cut into strips
Then add brown sugar and continue to fry, stirring until the sugar melts and the onions turn brown.
1 tbsp brown sugar
Turn the burgers over to the other side and fry for another 10 minutes.
Cover each burger with a slice of cheese and fry in air for 1-2 minutes until the cheese melts.
4 slices of cheese
Spread BBQ sauce on two bun halves. Place lettuce on the bottom half.
4 burger buns, 4 tbsp BBQ sauce, 1 cup lettuce
Top with a cheese burger.
Top the burger with caramelized onions. Cover with the other half of the bun, brushed with sauce and serve.