Heat the oven to 200C/180C fan/gas 4. Line a large, high-sided roasting tin with baking parchment, then arrange the potato and fennel slices in the tray, layering them in overlapping rows alternately until they cover the whole tin and doubling up the layers if you have leftover slices. Season with salt. Brush over the vegetable oil, then roast for 15 mins, or until the edges of the veg have started to crisp up and caramelise.

Meanwhile, combine the garlic, palm or brown sugar, fish sauce, lime juice, coriander stalks and 75ml hot water in a small bowl.

Lightly season the inside and outside of the fish with salt, then put the lemongrass inside the cavities. Brush the fish lightly with oil. Remove the veg from the oven and lay the fish on top. Pour a little of the sauce inside each cavity, then the rest over the skin. Cover with foil and bake for 15 mins until the fish is nearly cooked and the flesh flakes easily.

Remove the foil for the final 10 mins of cooking. Heat the grill to high. Slide the fish under the hot grill briefly until the skin has browned and blistered. Scatter over the coriander leaves, spring onions, chilli and lime slices, then serve.