Begin by heating half of the olive oil in a large Dutch oven or heavy-bottomed pot over medium-high heat. Season the chicken pieces generously with kosher salt and freshly ground black pepper. Add half of the chicken to the pot, searing the pieces on all sides until lightly browned but not fully cooked. Transfer the browned chicken to a plate and repeat the process with the remaining chicken, adding a little more oil if needed. Set all the browned chicken aside.

In the same pot, add the remaining olive oil. Add the sliced onions and season lightly with salt. Lower the heat to medium, cover the pot, and allow the onions to sweat for about 5 minutes, stirring occasionally. Remove the lid, then continue to cook the onions uncovered, stirring frequently and scraping up the flavorful browned bits (fond) from the bottom of the pan. Cook until the onions become tender and golden, about 10 more minutes.

Add the minced garlic to the softened onions, followed by the tomato paste, adjika if using, and crushed Aleppo chili pepper. Stir thoroughly to combine and cook for about 1 minute to develop the flavors.

Pour in the diced tomatoes along with their juices. Increase the heat and bring the mixture to a gentle boil. Taste and season the sauce with a pinch more salt if necessary. Return the browned chicken pieces along with any accumulated juices to the pot. Lower the heat to maintain a steady simmer, cover the pot, and cook until the chicken is fully cooked through—about 10 minutes, or slightly longer if using thighs. The internal temperature should reach 165°F when checked with an instant-read thermometer.

Once the chicken is tender and the sauce is rich and flavorful, taste and adjust the seasoning as needed. Stir in the chopped cilantro just before serving for a burst of freshness.

Serve the Chakhokhbili hot, accompanied by fluffy white rice, crusty bread, or warm flatbread to soak up the delicious, savory sauce.