Start by preheating your oven to 425°F and lightly greasing a baking sheet. This ensures the cakes cook evenly after pan-frying. Meanwhile, gather and prep all ingredients so everything is ready to go.
In a large bowl, whisk together egg white, Greek yogurt, lemon zest, salt, and celery seed. This mixture acts as the binder and flavor base. It should be smooth and slightly thick.
Pulse the sourdough bread in a food processor until it forms coarse crumbs. Then, flake the cooked salmon into small pieces using a fork. Make sure there are no large chunks for even mixing.
Add breadcrumbs, salmon, shallot, and capers into the yogurt mixture. Mix everything gently until well combined. Shape the mixture into evenly sized patties, about 8 cakes.
Heat oil in a skillet over medium-high heat and cook the cakes until golden brown on each side. This takes about 2–3 minutes per side. Transfer them to the baking sheet once browned.
Bake the cakes for 5–7 minutes until fully cooked through. While they bake, mix yogurt, lemon juice, mayo, dill, relish, mustard, and pepper to make a light tartar sauce. Serve warm with sauce.