To make the fish cakes: Spread 1/2 cup breadcrumbs on a plate and set out another empty plate.
In a medium bowl, thoroughly combine fish, eggs, onion, 2 tablespoons sour cream, 2 teaspoons lemon juice, 1/2 teaspoon mustard, 1/2 teaspoon salt, cayenne pepper and the remaining 1/2 cup breadcrumbs.
To form the cakes, grab a golf-ball-size handful of the fish mixture and squeeze it together with your hands just enough for it to come together (discard any excess liquid). Slightly flatten the ball, coat the outside with a light layer of the breadcrumbs on the plate and then place the fish cake on the empty plate. Repeat with the rest of the fish mixture.
Set a large sauté pan over medium-low heat and add a thin layer of olive oil to cover the bottom. When the oil is hot, cook the fish cakes (in batches if necessary) until golden brown, 3 to 5 minutes per side. Serve warm with dipping sauce.