Peel the potatoes and dice them. Put them in a pan of cold water, bring to the boil and simmer for 15 minutes or until tender. Drain well. Alternatively bake the potatoes in the oven until tender, let them cool a bit, then scoop out the flesh into a bowl.
Mash the potatoes with the butter or spread, using a potato masher. It should have no lumps.
Chop the crayfish tails and spring onion into very small pieces. Add these to the potato.
Add the herbs and the grated lemon. Season to taste.
Using your hands, form the mash mixture into 4-6 balls and then press them into 1.5 cm thick patties.
Coat the patties in flour (or breadcrumbs) then fry them in hot oil over a medium heat until golden on each side, turning them once.
Serve warm or at room temperature.