Peel the potatoes and cut them into 1/2-inch cubes. Place them in a large pot, cover with water, add salt, and bring to a boil. Cook the potatoes for 12–16 minutes, or until tender. Check doneness with the tip of a knife. Drain, reserving a little cooking water if you want a creamier mash.

Slice the leek in half lengthwise, wash thoroughly, then cut into thin half-moons. Chop the onion. Add the onion and leek to a large skillet with a drizzle of olive oil. Sauté for 15 minutes.

When the potatoes are cooked, mash them with a potato masher. Add a little reserved cooking water and a pat of butter or a splash of milk for creaminess. Season lightly with salt and pepper, and mix well. Cover and set aside.

Preheat the oven to 430°F (220°C) on broil. Pour the cream over the leek mixture, then add the red wine vinegar and mustard. Tear the wild garlic leaves and add most of them (save a few for serving if you like). Season with pepper. Cook for 2 more minutes, or until the sauce thickens slightly.

Transfer the leek mixture to a baking dish, add the hot-smoked salmon flakes, and mix. Spread the mashed potatoes on top. Sprinkle with Swiss cheese. Bake in the top of the oven for 3–5 minutes, or until golden and bubbly.

Meanwhile, make a vinaigrette by mixing a drizzle of olive oil, balsamic vinegar, and mustard in a salad bowl. Just before serving, add salad greens and toss well. Cut the salmon parmentier into portions and plate. If you reserved any wild garlic leaves, tear them over the top. Serve the salad on the side.