Grate the Onion: Use a food processor or hand grater to grate the onion. Place the grated onion in a fine mesh strainer over a mixing bowl and squeeze out all the juice. Discard the pulp.
Prepare the Marinade: Mix the remaining ingredients (excluding the meat) in a bowl to form the marinade. Set aside.
Cut the Meat: On a cutting board, cut the filet mignon into equal-sized pieces, about 3-4 inches long, yielding approximately 6 pieces.
Butterfly the Meat: Make the first horizontal cut one-third of the way down from the top, extending about three-quarters of the way across the piece. Do not cut all the way through. Flip the piece over and make the second horizontal cut from the opposite side, again one-third of the way down from the top, extending about three-quarters of the way across. The filet piece should now be in an S shape, which when opened will be about one-third of its original thickness. Repeat for the remaining filet pieces.
Tenderize the Meat: Using the blunt edge of your knife (cutting edge pointing up), pound the knife across each butterflied filet piece to tenderize. The meat should be about half its original thickness after this process.
Marinate the Meat: Place the tenderized filet pieces in a shallow pan and pour the marinade over them. Cover and refrigerate for at least 2 hours, preferably overnight.
Prepare the Grill: Before grilling, ensure your charcoal or gas grill is hot and ready.
Skewer the Meat: Remove the filet pieces from the fridge and place them on a cutting board. Using flat and long skewers, gently push a skewer through each piece of meat until firmly secured.
Grill the Kebabs: Place the skewers on the grill and cook for 6-8 minutes, rotating frequently for even cooking. The kebabs will firm up and no longer droop over the skewers as they cook.
Serve: Serve Kabob Barg with grilled vegetables, flatbread, or steamed basmati rice.