Yogurt sauce: In a small bowl, combine the yogurt, lemon, maple syrup and fresh herbs. Refrigerate (serve at room temperature). The sauce may be prepared a day in advance.
Cabbage preparation: In a saucepan over medium-low heat, melt the butter and sweat the leeks for 6-8 minutes, without browning, while stirring. Season to taste. Let cool.
In a large casserole with boiling salted water, blanch the cabbage leaves for 30 seconds, two at a time, and then immediately plunge them into a large bowl of ice water. Gently blot the leaves with paper towel and cut the big rib in the middle.
Preheat oven to 190ºC (375ºF).
Salt and pepper the salmon. On a work surface, layer two cabbage leaves, head to tail. Top with 1/4 of the leeks, and a salmon steak. Fold a corner of the cabbage leaf on the salmon and roll it all, being careful to finish with the folded side underneath. Repeat with the remaining ingredients.