In a large skillet, add two tablespoons of olive oil and the diced onion, saute until the onion is soft and translucent, then add the chopped steak, cook until the meat is browned.
While the steak is cooking, cut the bell pepper, half remove the seeds. Then rub a teaspoon of olive oil over the bell peppers, both the inside and out.
Place the bell peppers, face down on the air fryer tray, or in the air fryer basket. Set the temperature to 400 degrees F for 5 minutes. Flip the pepper after about 2 minutes, so both sides get roasted.
Then fill the bell pepper up with the steak/onion mixture.
Sprinkle some mozzarella cheese on top of the filling.
Set the filled peppers back on the air fryer tray or basket, and set the temperature to 400 degrees F for 3 minutes. As soon as the cheese is melted, remove from the air fryer.