Pat salmon dry with a paper towel and sprinkle with salt and freshly ground pepper. Place skin-side down on a grill over medium or indirect heat, close lid and cook until flesh flakes, 8-10 minutes. You can also place on a baking sheet and cook in a 300 F. oven for 20-25 minutes. Remove and allow to stand while you make the bread.
Mix garlic with soft butter and butter one side of bread slices. Place onto medium heat grill, butter side down, until toasted. Remove to serving plates. Top bread with some watercress. Flip salmon over, remove skin and discard. Cut salmon into pieces and place on piece on top of watercress. Top with a dollop of dressing. Serve with a lemon slice, for drizzling.
NOTE: How to Make Creme Fraiche at Home: In a medium jar, stir together 1 cup of heavy, 35% whipping cream (not ultra-pasteurized) and 1 Tbsp. buttermilk. Set a lid on top, but don't twist to seal it (to allow air to escape, but to keep dust and things out). Set on your counter or window sill for 12-24 hours, or until the mixture resembles stirred sour cream. The time it takes to that point will vary depending on the temperature of your room. Mine took about 20 hours. At that point stir the mixture well, then transfer to the fridge, again, topping with a lid but not twisting to seal it, as it will continue to process). Refrigerate a further 6 hours or longer, or until it is the consistency of cream cheese.