Halve the yellow onion. Dice one half and roughly chop the other into larger strips.
Place a medium saucepan over medium heat and add a tbsp of the sunflower oil. Add the diced half of the onion and fry until softening and turning translucent. Add the moong dal and 1/2 a teaspoon of the ground turmeric. Stir to combine and cover with water. Place the lid on and bring to a simmer before reducing the heat to low.
Allow the lentils to cook, stirring occasionally, until the lentils are completely cooked and the mixture resembles a very loose porridge.
While the lentils cook, place a large saucepan over a medium heat. Before adding any oil, toast the remaining turmeric, cumin seeds, coriander seeds, garam masala, fenugreek powder and chilli powder in the dry saucepan until smoking slightly and very fragrant. Transfer the spices to a mortar and pestle or a spice grinder and grind well. Set aside.
Add the remaining 3 tbsp sunflower oil to the hot pan, followed by the roughly chopped half of the onion. Fry until the onions are turning translucent but don't allow them to brown.
Add the ginger, garlic and chillis and stir to combine before cooking for a few minutes. Add the spices, chopped tomatoes and light brown sugar. Stir to combine. Reduce the heat to low and allow the tomatoes to cook, stirring regularly, until no longer holding their shape.
The lentils should now be cooked. Add half of the lentils to the tomato mixture and stir to combine. Remove the remaining half of the lentils from the heat and use a stick blender to puree them before adding to the curry.
Finally, add the roasted romanesco to the curry and stir to combine. Add the juice of half the lime and season to taste with flaky sea salt. Top with the chopped coriander before serving with steamed basmati rice.