Drain the salmon, remove bones and skin if desired. Flake into a bowl, then add 1/2 cup of the cracker crumbs, egg, onion, mayo, mustard, lemon juice, salt, and pepper. Mix gently just until combined.

Form the mixture into 4 patties about 3 inches wide and 1/2 inch thick. Chill for 5 to 10 minutes if they feel soft.

Spread the remaining cracker crumbs on a plate and coat each patty on all sides, pressing gently so the crumbs stick.

Pour enough oil into a skillet to reach 1/4 inch deep and heat over medium until shimmering. Fry the patties 3 to 4 minutes per side until deep golden. Drain on paper towels.

Transfer to a plate, sprinkle with a little flaky salt if you like, and serve right away with lemon wedges and your favorite sauce.