Heat the oven to 180°C/Fan 160°C/Gas 4. Scrub the potatoes and cut them into 1 cm-thick slices. Scrub and trim the carrots. Cut them into roughly 8 finger-length chunks each.
Tip the veg into a bowl. Pour over 1 tbsp olive oil. Scatter over 2 tsp cumin seeds and 1 tsp ground turmeric. Dust with salt and pepper. Toss together to coat all the veg in the oil and spices. Tip the veg onto a tray in a single layer. Slide the tray into the oven and roast for 20 mins.
Trim and slice the leek in half lengthways. Wash out any dirt. Finely slice it. Zest the lemon and cut it into wedges. Chop the butter into cubes. Peel and thickly slice the red onion.
When the veg have roasted for 20 mins, add the red onion and cherry tomatoes to the tray. Roast for a further 20 mins till the veg are tender and a little charred.
When the veg have 10 mins left to cook, set a frying pan on a medium high heat. Rub 1 tsp olive oil into the skin of each whiting fillet and place them in the pan, skin-side down. Cook for 2 mins. Add the cubed butter, 2 tsp each of garam masala and black mustard seeds, the lemon zest and the leek. Swirl the pan to melt the butter and coat the fish. Pop a lid on the pan. Cook over a medium heat for 4-5 mins.
Place a handful of baby leaf spinach on each plate. Divide the golden veg chips between the plates and top with a spiced whiting fillet. Spoon over the butter and leek sauce. Serve with wedges of lemon for squeezing.