1. Scrub the salad potatoes, then tumble them into a large pan and cover them with cold water. Add a good pinch of salt and pop the pan on the hob. Bring to the boil, then reduce to a simmer. Cook the potatoes gently for 20-25 mins, till tender in the middle. When the potatoes are cooked, drain them and set aside to steam dry in the colander.
2. While the potatoes cook, peel and thinly slice the red onion. Peel and thinly slice the garlic cloves. Thinly slice the gherkins. Rinse the capers under a cold tap. Thinly slice the radishes. Add the onion, garlic, gherkins, capers and radishes to a large dish or bowl.
3. Finely chop the mint and parsley leaves. Scoop half of the herbs into a separate bowl, and set aside for later. Tip the remaining half of the herbs into the dish with the red onion. Grate the zest of 2 lemons into the dish and squeeze in the juice (save the other lemon for later). Pour in 3 tbsp olive oil and add a good pinch of salt. Stir the ingredients in the dish together to make a dressing.
4. When the potatoes are cool enough to handle, but still warm, slice them in half and add them to the dish with the lemon and herb dressing. Toss the potatoes with the other ingredients in the dish, till evenly coated. Arrange on a large serving platter and set aside.
5. Grate the zest of the remaining lemon into the bowl with the remaining herbs. Add a pinch of salt and pepper and stir well. Remove the packaging from the salmon and carefully open the cavity in the middle. Use a teaspoon to spoon the herbs into the salmon, trying to spead them out as much as possible. Close the salmon tail back together. Drizzle the outside of the salmon with 1 tbsp olive oil and rub with a good pinch of salt and pepper.
6. If using a barbecue, make sure that the coals are white and the flames have died down. If using a frying pan or griddle, warm to a high heat. Carefully add the salmon to the hot barbecue or pan and fry it for 7-8 mins per side, till the skin is dark and crispy and the flesh light pink and flaky. Try and turn the salmon just the once, which will help it stay in one piece.
7. When the salmon is cooked, arrange it on top of the potato salad and serve.