Season the beef cheek with salt, black pepper, and granulated sugar. Let rest for 10 minutes. In a blender, puree the vinegar, ginger, and garlic.

Place the seasoned beef cheeks in a bowl, pour the liquid over the meat, and mix to coat. Cover with plastic wrap and let marinate in the refrigerator overnight or up to 24 hours.

Remove the beef cheeks from the refrigerator. Drain, reserving the liquid, and bring the meat to room temperature for about 1 hour. Transfer the reserved marinade to a small saucepan and cook over low heat until reduced by half. Set aside.

In a separate small saucepan, bring the stock to a boil. Preheat the oven to 325F. Add 1 tablespoon of the oil and ½ tablespoon of the butter to a sauté pan and place over high heat. Add one-quarter of the meat and sear on both sides until golden, about 4 minutes, then transfer to a plate with a slotted spoon and set aside. Repeat this process, adding another 1 tablespoon of the oil and ½ tablespoon of the butter to the pan before adding another one-quarter of the meat, until all the meat has been seared.

Wipe the pan clean, then return to the stove over high heat and add the remaining 3 tablespoons of oil. Add the onion and bay leaf, and cook until the onion is translucent, stirring occasionally to avoid browning , about 4-5 minutes. Remove the pan from the heat and add the ras el hanout. Stir to coat the onions and bloom the spices for about 1 minute. Add the brown sugar and the reduced marinade liquid, and stir to combine.

Transfer the onion mixture to a 6-quart baking dish and top with the seared beef cheeks. Pour the hot stock over the beef cheeks. Then cover the dish with aluminum foil. Bake until the meat is fork-tender and has fully broken down, about 2½ hours. Remove the cheeks from the oven and uncover. Add the sliced serrano chile and let rest, uncovered, for 1 hour. 

To serve, transfer the cheeks to a large serving bowl or tagine and top with the prune and apricots. Garnish with cilantro leaves. Serve hot with a side of rice, couscous, or mashed potatoes.