1Place salmon fillet on a cutting board and flake it into bite-sized pieces with a fork, removing any remaining pin bones as you go.
2In a mixing bowl, combine the flaked salmon, panko, egg, diced onion, dill, dijon mustard, salt, and black pepper. Fold gently until just combined—don't overmix or the cakes will be dense.
3Divide the mixture into 8 equal portions and gently shape each into a flat patty about 0.75 inches thick. If the mixture feels too wet, add another tablespoon of panko.
4Heat oil in a skillet over medium-high heat until it shimmers. Working in batches, carefully place cakes in the pan and cook for 3–4 minutes on the first side until golden and crispy.
5Flip and cook for another 3–4 minutes on the second side until cooked through and golden brown.
6Transfer to a paper-towel-lined plate. Cut the lemon into wedges and serve with the cakes.