Put all the ingredients for the lemony herby dressing in a small food processor and blend until smooth
In the bottom of a round oven-safe pan, (12 inches) drizzle 2 tablespoons of olive oil and sprinkle ¼ teaspoon of Maldon salt. Spread it with your hands.
Arrange the cabbage evenly, then spread the sliced onion on top.
Drizzle the remaining 2 tablespoons of olive oil on top and sprinkle with the remaining ¼ teaspoon of Maldon salt.
Cover with wet parchment paper and aluminum foil.
Bake at 420°F/220°C for 45 minutes.
Remove the cover and switch to low broil for 10 more minutes, or until nicely golden brown.
Use a tablespoon to spoon the green dressing evenly over the cabbage.
Garnish with a touch of sumac.
