Heat oven to 180°C/350°. Put 2 tbsp olive oil in a baking tray and place in the oven to get hot. Bring a large pot of salted water to the boil. Par boil the potatoes for about 10 minutes or until you can put a knife in quite easily.
Whisk 4 tbsp olive oil, harissa, lemon juice and garlic together and spread half on the bottom of a baking dish. Season the salmon with salt and pepper on both sides then put in the baking dish skin side up (if it has skin).
Pour over the remaining sauce and spread then top with the lemon slices and put in the oven. Drain the potatoes, get the tray of oil out the oven and spread the potatoes on it. Sprinkle with salt and pepper and put in the oven.
After 15 minutes take the salmon out of the oven and cover with foil. Turn the heat up to 225°C/440°F and cook the potatoes for another 15 minutes. Serve the salmon with the potatoes.
Nutrition information is an estimate and will vary depending on the exact ingredients used.