In a medium-size nonstick skillet, heat vegetable oil over medium-high. Add onion, finely chopped, and season with coarse salt and ground pepper. Cook until onion is tender, 3 minutes. Transfer to a bowl and let cool. Stir in 1/4 cup breadcrumbs, egg, lemon zest, and lemon juice. Gently fold in poached salmon.
Season 1/2 cup plain dried breadcrumbs with salt and pepper. Divide salmon mixture into 8 portions and dredge in breadcrumbs. Form into 3/4-inch-thick cakes, patting crumbs to adhere.
In a medium skillet, heat 1 tablespoon oil over medium. In two batches, cook salmon cakes until golden brown, 3 to 5 minutes per side, adding 1 tablespoon oil for second batch. Serve with yogurt sauce.