Mix the mayonnaise and mustard, divide into 2 small bowls, then stir the onion into one bowl.
To assemble, spread a layer of the onion mayonnaise over each wrap and add 2 slices of smoked salmon, some sliced hard-boiled egg and a generous helping of spinach to each.
Roll up tightly. The wraps can be made several hours ahead and kept covered in the fridge. To serve, cut each wrap on the diagonal into 2 pieces.
