Place eggs, milk and melted butter in a bowl and whisk until combined. Sift flour, baking powder and salt into a bowl. Make a well in the centre, gradually add egg mixture and beat until smooth. Allow to rest for 2 hours (or overnight, if possible).
Heat a non-stick frying pan and ladle 4 tablespoons of batter into the pan. Sprinkle 1 tablespoon of spring onions over pancake. Turn pancake and cook on the other side. Transfer to a plate and keep warm while cooking remaining pancakes.
For the lemon butter, heat the juice of two lemons in a small pan over medium heat. Whisk in 125g unsalted butter in small pieces until melted and combined. Season with salt and freshly ground black pepper.
To serve, place a pancake on each plate, top with 1 cup of baby spinach, 4 slices of gravlax and a pile of cucumber ribbons. Spoon lemon butter around, but not on top of, the pancake.