Blitz the spices until coarsely ground in a spice mill, or use a pestle and mortar and a bit of elbow grease, then mix with the salt and oil to make a paste.
Using a sharp knife, slash the skin of the fish deeply a few times, but not all the way through, to help the marinade to soak in. Rub the spice paste all over the fish, being a little more generous on the skin side, rubbing it well into the cuts you have made. Leave to marinate for 30 minutes, covered but out of the fridge.
For the dressing, mix everything together in a bowl and season with salt and black pepper. Set aside.
Fire up the barbecue; one side on hot and the other on medium-hot. You can start cooking the potatoes immediately on the hot side, but you need to have pre-heated the barbecue on medium-hot in readiness for the fish which is grilled later.
Start the potatoes first as they take the longest to cook. Add the oil and mustard seeds to a grill proof frying pan or cast iron skillet (with no plastic handles) and set over the high heat. Once the seeds start to pop add the butter and turmeric, stirring as it melts. Add the onion and fry for a few minutes until it starts to soften, then add the garlic, stirring to mix. Tip in the potatoes, season generously with salt and pepper and fry for about 20 minutes until they are tender, keep an eye on them and stir from time to time to stop them sticking. Grilling with the lid down creates and even, oven-like heat, which will speed up cooking.
Meanwhile, transfer the salmon to a barbecue fish basket and cook, skin-side down first, over the pre-heated medium-hot side of the barbecue for around 12 minutes, turning over halfway through, until deep golden and crispy. Transfer to a large serving platter, using a palette knife to release the fish from the basket and rest on the warming rack until the potatoes are cooked.
To serve, pile the potatoes around the fish. Drizzle over a little of the dressing and then scatter over the coriander. Serve the rest of the dressing on the side, along with the lemon wedges to squeeze over.