For the crepes:
In a medium bowl, whisk together milk and eggs. Add flour and salt; mix until very smooth. Cover and refrigerate for at least half an hour.
Heat a fry pan over medium heat. Add butter and swirl in to coat entire pan. Ladle about 2 oz. worth of batter into center of pan; move pan around to quickly spread batter.
Cook until edges of crepe look cooked, then carefully turn over with a spatula or flip in pan. Briefly cook the other side. Remove crepe; return pan to stovetop and repeat process with remaining batter.
For the filling:
In a small bowl, mix the onions and red wine vinegar. Set aside.
In another small bowl, mix the goat cheese and dill; season to taste with salt and pepper. With a spatula, spread some of the goat cheese mix in a thin layer over each crepe.
Place 3 slices smoked salmon in a single layer overtop of the goat cheese spread. Add some of the marinated red onion, a small handful mixed greens, and roll or fold into quarters. Serve immediately.