For the crepes: Combine the milk, flour, sugar, salt, eggs and 2 tablespoons of the butter in a blender and blend until smooth, about 20 seconds. Let the batter rest at room temperature for at least 15 minutes before using or transfer to a container with a lid and let rest in the refrigerator for up to 1 day.
For the lemon creme fraiche: Whisk together the creme fraiche, lemon zest and juice and some salt and pepper in a small bowl. Set aside.
For the herbed ricotta and topping: Put the ricotta, chives, dill, parsley, lemon zest and some salt and pepper in a medium bowl and work together with a fork.
Heat a 10-inch nonstick skillet over medium heat. Lightly brush with some of the remaining melted butter. Add 3 to 4 tablespoons of the crepe batter and swirl the skillet so the batter completely covers the bottom. Cook until the underside of the crepe is golden brown, 2 to 3 minutes. Loosen the edge of the crepe with a rubber spatula, then with your fingertips, quickly flip. Cook 1 minute more. Slide the crepe out of the skillet and repeat with the remaining batter, coating the skillet with butter as needed. Stack the finished crepes directly on top of each other.
Lay 8 crepes on a flat surface. Smear about 1/4 cup herbed ricotta in a wide strip down the center of each. Top with 2 or 3 slices of salmon, then gather the sides of each crepe around the filling, leaving most of the filling exposed. Scatter some onion slices and capers over the filling, then drizzle some lemon creme fraiche on top. Scatter some dill fronds on top and serve.