Drain canned salmon (or tuna) thoroughly — squeeze out every drop of liquid. Wet patties won't bind and won't crisp.
In a bowl, combine drained fish, breadcrumbs, egg, diced onion, Dijon mustard, and salt. Mix with your hands or a fork until the mixture holds together when squeezed. Don't overwork it.
If too wet, add more breadcrumbs 1 tbsp at a time. If too dry, add a splash of olive oil.
Form into 8 patties, about ½ inch thick. Flatten them — thicker patties won't cook evenly.
Heat olive oil in a skillet over medium-high heat until shimmering.
Cook 3–4 minutes per side without moving until deep golden brown. Flip carefully.
Serve with lemon wedges and Greek yogurt mixed with Dijon as a dipping sauce.