In large saute pan over medium high heat, saute onions in butter until golden brown, about 5 minutes. Remove from heat and set aside.
In large bowl, combine salmon, cajun seasoning, lemon juice, crushed saltines, salt, pepper, egg, and the cooked onions. Stir well to combine. Form into four patties.
Place cornmeal in shallow dish and coat each patty on both sides.
Using same pan you cooked the onions, heat the oil over medium high heat. Add the salmon patties and cook until crisp on both sides, about 5-8 minutes each side. Serve hot or cold.