Preheat oven to 350 degrees Fahrenheit.

Heat vegetable oil in an ovenproof skillet over medium-high heat.

Fry thinly sliced onion until crisp and golden.

Drain the fried onions on paper towels and reserve the oil in the pan.

Mince parsley, cilantro, and dill in a food processor.

Combine the minced herbs in a bowl with the fried onions, thinly sliced romaine lettuce, thinly sliced scallion greens, bread crumbs, turmeric, and barberries (if using).

Add lightly beaten egg and mix until just combined.

Season the herb mixture lightly with salt and pepper.

Make a lengthwise slice down the middle of each beef tenderloin to about 1/2 inch from the bottom to open it like a book.

Season all sides of the beef tenderloins with sumac and a mix of cumin, coriander, cinnamon, and cardamom.

Put the herb mixture into the opening of each tenderloin, filling it generously.

Tie the tenderloins with twine at 1-inch intervals to secure the stuffing.

Return the skillet to high heat.

When the pan is hot, add the stuffed tenderloins, turning to sear on all sides until browned.

Transfer the skillet with the seared tenderloins to the preheated oven.

Cook until an instant-read thermometer inserted into the thickest part of the meat (not the stuffing) reads 130 degrees Fahrenheit for medium-rare, approximately 20 to 25 minutes.

Transfer the cooked tenderloins to a warm platter, cover lightly with foil, and allow them to rest for 10 minutes.

Cut the tenderloins into slices, discarding the butcher's twine, and serve immediately.