Soak Saffron strands in the lemon juice. Set aside for 10 minutes, then add to the mayo mixing well to combine. The mayo should be fairly loose - add a little more lemon juice if necessary.
Break up the smoked fish into small pieces.
Chop beans and asparagus into halves or thirds.
Combine all fish, vegetables and fresh herbs, retaining some of the coriander for garnish. Toss all gently together to combine and arrange on a platter which has been lined with baby spinach leaves.
Drizzle with the saffron mayo, then sprinkle with croutons and chopped coriander.