If using a gas grill, preheat the three burners. Place a plancha on the grill, and let it heat to about 425-450F (220-230C).

Rub the salmon all over with a light coating of oil.

When the grill is hot, place the salmon on the plancha (you can also place it right on the grill), skin-side down. Put the butter in a heat proof pan and place it on the grill as well. Close the grill and cook the salmon until it is translucent through, which should take 8 minutes. Check the salmon; if it is just about but not quite entirely cooked, you can leave it on the grill another minute or two, remembering that it will continue to cook even when you remove it from the grill.

To remove the salmon, slip a metal spatula between the meat and the skin and place the meat on a warmed plate. Season it with salt and pepper. Let it sit for a few minutes. Remove the skin and reserve.

Mince the tarragon and stir it into the melted butter, then stir in the lemon juice to taste.

To serve, divide the salmon steak in two, and place one piece in the center of each of two warmed plates. Drizzle each piece with half the butter mixture. Cut the salmon skin in two, and balance it at an angle on the salmon. Serve immediately (here with a leeks that have been cut in two, lengthwise, rolled in olive oil with salt, and grilled for 8 minutes along with the salmon).