Prepare the cauliflower by removing any outer green leaves and stems, keep the core attached, as this will keep the steak intact. Place the trimmed cauliflower heads on the flat side down with the florets face up. Slice into 1-inch steaks. You should be able to get 2 steaks with some excess florets from the sides.
Grease a baking tray and place the cauliflower steaks. Preheat your oven to 180°C.
In a mixing bowl, combine all of the ingredients for the marinade except the chaat masala. Using a spoon or pastry brush gently brush and coat the cauliflower steaks with the marinade. Set the chaat masala aside as it is not required until the cauliflower steaks are cooked.
Place the marinated steaks on a baking tray, tumble the excess cauliflower florets in the now-empty bowl (which you made the marinade in) and tip the florets onto the tray along with the steaks. Place the tray in your hot oven for 1 hour, 30 minutes on each side.
Whilst the cauliflower steaks are baking, make the channa masala. In a saucepan, heat the 2 tbsp vegetable and add in the cumin seeds. Fry the seeds in the hot oil until they sizzle, then add in the diced onion along with the 1/2 tsp salt and fry for 6-7 minutes on medium heat until the onions have softened.
Add the garlic and ginger pastes into the softened onions and fry for 1 minute then stir in the chilli, coriander, cumin, turmeric, paprika and channa masala powders. Fry on a low heat for 2 minutes.
Add the tomato paste and 1/2 cup of water into the spiced onion base. Stir well and leave the base to simmer for 1-2 minutes to form a sauce then Add in the chopped tomatoes and the sugar, stir well and cover with a lid. Simmer on a low heat for 5-6 minutes stirring occasionally encouraging the tomatoes to break down.
Once the cauliflower steaks have roasted for 30 minutes. Gently flip the steaks, drizzle over a little oil and allow to roast for 30 minutes on the other side.
Returning back to the channa masala, stir in the drained chickpeas and the remaining 1/2 cup of water and allow to simmer on a low heat for 10 minutes. Once the chickpeas have simmered for 10 minutes, it should have thickened to form a masala, stir in 1/3 of the chopped coriander, taste to check the seasoning and leave aside.
When the cauliflower has roasted for a total of 1 hour, remove it from the oven and leave it aside whilst you assemble the dish.
On serving plates, spoon on equals amounts of the channa masala and top each plate with a cauliflower steak. Sprinkle each steak with an equal amount of the chaat masala. Garnish the dishes with the remaining chopped coriander and pomegranate seeds.