In a medium bowl, whisk together egg, mayonnaise, lemon juice, chilli flakes, tarragon and spring onions. Gently stir in crabmeat, being careful not to break up meat. Gradually mix in cracker crumbs, adding until desired consistency is achieved.
Heat butter in a frypan over medium heat. Form crab mixture into 4 large or 8 small patties. Place patties in frypan and cook until golden brown, about 5 to 6 minutes on each side.