Heat the oil in a small pan and cook the onions until soft and translucent. It´s okay if they brown a bit; that will enhance the flavor.
Soak the bread roll in water and once it's completely soaked, wring out excess water.
Now mix all remaining ingredients - EXCEPT the feta cheese - and knead until combined.
Divide the ground beef mixture into either 4 bigger or 6 smaller parts and form each into a flat circle.
Divide the feta cheese between all meat circles and carefully close each piece so no cheese will leak out later.
Place bifteki into your air fryer and cook at 400°F for 12-14 minutes.
Serve immediately.
Notes
You can substitute the bread roll with either one slice of sandwich toast or 2 heaping tablespoons of bread crumbs.
With ground beef, you can also cook it to only medium doneness if you prefer it to be even juicier. Carefully check on bifteki after 10 minutes or use a thermometer to cook until 161°F.
It's okay to place the bifteki a bit tight in your air fryer. They will shrink when cooked.
You can also use ground pork or ground lamb - a mixture of ground pork, ground lamb, and/or ground beef.
If you don’t like onions or have fussy kids, substitute with 1 heaping teaspoon of onion powder.
You can add minced garlic if you like it. Also, a dash of fresh lemon juice is an excellent addition.
If you have it at hand, use fresh oregano.
You can also play around with the filling. For example, why not try mozzarella, parmesan cheese, or gouda?
These bifteki can also be made in a pan, on the grill, or - if you want to make a larger amount - in the oven.