Place the rounds of potatoes in a pan of cold salted water and bring to a boil. Cook for about 10 to 15 minutes, or until while piercing with a knife you find no resistance. The potatoes need to stay intact and not falling apart.
Meanwhile line a cookie sheet with aluminum foil. Put 2 to 3 slices of lemon on the aluminum foil and place a salmon over it, repeat with each salmon fillet. Sprinkle generously with salt and pepper. Top with fresh thyme.
Place in the hot oven and bake for 15 to 20 minutes, or when the fillets are done. Remove the lemons slices on the bottom of each fillet, and place it on a warm plate.
Rinse the arugula and butter lettuce and place in a large salad bowl. Add the mushrooms slices.
Whisk the vinaigrette - 3 tablespoon olive oil, 1 tablespoons red wine vinegar, salt and pepper. Gently fold the dressing ( last minute at serving time or your salad will wilt) into the salad to cover all the leaves.
In a dinner plate arrange the potatoes, the green salad and the salmon.
Drizzle a little bit of olive oil, couple of grinds of black pepper at serving time over the salmon fillets and the potatoes