To prepare the dressing, combine the lemon juice, olive oil, water, mustard, maple syrup, garlic, and fresh dill in a blender and blend until smooth. Taste and adjust the seasoning, if needed.
To assemble the salad, fill a large bowl or plate with the mixed greens. Top them with small pieces of smoked salmon, slices of avocado, and the red onion. Drizzle the dill dressing over the top just before serving.
Leftover salad can be stored in the fridge for up to 4 days, but it keeps best if the leaves haven't touched the dressing, so store the dressing separately for best shelf life.