Pat salmon dry. Pat dry again. (Wet fish will not sear). Sprinkle both sides of fish with salt.
Heat oil in a stainless steel or cast iron pan over medium-high heat, until oil begins to shimmer.
Set salmon, skin side down, in hot pan. Sear, without touching or trying to move salmon, for approximately 4 minutes per ½ inch thickness. Salmon will begin to cook from the bottom up- turning from pale pink to whiter and more opaque. Salmon is ready to flip when the fish has cooked approximately ¾ of the way through.
Use a thin metal spatula to flip salmon. If salmon sticks to the pan, allow it to cook another 30 seconds and try again until it's loosened.
Once salmon is flipped, sear it for about 1 minute. Remove salmon from pan. Flake the fillets apart.
Toast bread, and set aside. Mix lemon juice and all remaining Dijon sauce ingredients together.
Layer the flaked salmon, a generous dollop of yogurt sauce, sliced tomatoes, and lettuce in between the slices of bread to make two sandwiches. Cut sandwiches in half if desired, and serve immediately.