To make the fire-roasted cabbage, drizzle the cabbage wedges with the sunflower oil. Use your hands to spread the oil all over to coat the outside. Season with the sumac, sage, salt, and pepper, making sure all sides get seasoning.
If you have a sous vide machine, place the wedges in a vacuum bag (depending on the size of the bags, one to two wedges will fit in each), and seal according to directions. Place the cabbage wedges in an immersion bath set at 180°F and cook for 4 hours.
If you don’t have a sous vide, roast the cabbage wedges in the oven with low heat to achieve a similar effect. Preheat the oven to 325°F. Place the prepared cabbage wedges on a rimmed baking sheet or in a baking dish that fits them snugly. Add ¼ cup of water to the baking sheet and cover it with foil. Bake for 1 hour, then turn the cabbage wedges over. If needed to prevent burning, add a splash or two more of water. Bake for another hour.
Once cooked, chill the cabbage in the refrigerator while you prepare the sauce.
To make the roasted chili-chestnut sauce, roast the bell pepper. You can do this over a wood or charcoal grill, on the stovetop, or in the oven. If using a grill, place the pepper directly on the grill before the initial flames die down and cook, turning occasionally, until it is blackened on all sides.
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