heat butter in skillet over med-high heat, add onion, saute till tender, let cool. combine shrimp, cooled onions,cajun seasoning, cracker crumbs, cayenne pepper, eggs, and milk. shape into patties; cover and chill for 2 hours. heat oil in large skillet over med-high, saute shrimp cakes in batches 4-5 minutes on each side or until golden brown, drain on paper towels, serve with sauce. for sauce combine ketchup, horseradish, lemon juice, tarragon, and lemon zest in bowl, cover and refrigerate, do this before you make the cakes enjoy