1. Heat a little oil in a frying pan and fry the onion until softened.
2. Put into a large bowl and stir in the chives, mashed potato and tuna. Season well. Shape into 8 patties and press firmly into the matzo crackers until well-coated.
3. Shallow-fry on both sides until golden, then drain on kitchen paper.
4. Mix the mayonnaise, tartare sauce and a few more chopped chives. Serve alongside the fishcakes with lemon wedges to squeeze over.