Can you substitute salmon or trout? We don't get Arctic Char in the UK so I have no idea what it tastes like!

I've only eaten Arctic char when I lived in the Canadian far north. I wonder if in the UK it's fished from the northern Scottish coast? It has a lovely delicate flavour and texture. I hope that the recipe isn't overpowering. It's gorgeous with as little additions as possible.

Yes! If you use trout, follow the cooking instructions as written in the recipe. If you use salmon, which will likely be thicker, you won't need to do the turned-over sheet pan trick to get the pan closer to the broiler, but you will need to cook the salmon longer — probably 5 to 6 minutes. Keep an eye on it. If it's getting too brown, you can finish the fillets in a hot skillet — just heat some oil, and sear the bottom side (the skin side) for a minute.