Make the sauce by combining all ingredeints together and refrigerate. Squeeze most of the lemon juice into the sauce and save a splash for the cakes.
Drain salmon well. Pick out any discoloured skin and bones. Add to mixing bowl and break apart with fork.
Tear a slice of white or brown bread into chunks and pulse in a food chopper until small crumbs. Add to salmon.
Using the tip of a sharp knife, slice celery and green onion into lengthwise strips, then chop into small pieces. Mince dill. Drain and chop capers. Add to salmon along with mayo, dijon, salt and pepper.
Squeeze a little lemon juice into mixture from lemon used for sauce.
Separate egg. Add yolk to salmon mixture, combone all ingredients well.
Beat egg white until thick and stiff. Fold gently into salmon mixture.
Add panko breadcrumbs to wide shallow plate. Season with salt and pepper.
Heat a wide non stick pan to medium heat and add 1 Tbsp oil.
Spoon salmon mixture into ¼ cup measuring cup and press gently. Drop onto panko crumbs. Sprinkle crumbs over top and press, shaping into thick round cakes. Using a wide thin spatula, carefully place 3-4 salmon cakes into hot pan.
Gently press cakes down with spatula to ensure a good golden crust. After about 1 minute when edges of crumbs at bottom are golden brown, carefully flip using a spatula and fork and fry other side.
Remove cakes to a plate and keep in a warm oven while cooking second batch. Add 2nd Tbsp of oil to pan after first batch. Wipe out any dark bits of panko after first batch if necessary.
Serve warm with sauce and garnish with extra dill if desired.