Make the dill sauce: In a small bowl, stir together the sour cream, mayonnaise, lemon juice, dill, Dijon mustard, salt, and pepper. Cover and refrigerate while you prepare the salmon cakes.
In a large bowl, combine the panko and milk. Let stand for 2 minutes so the crumbs soften slightly.
Add the drained salmon, egg, grated onion, dill, Dijon mustard, lemon zest, salt, and pepper. Gently mix with a fork just until evenly combined. Do not mash the salmon into a paste.
Shape the mixture into 8 equal patties, about 2 1/2 inches wide. If the mixture feels too wet to shape, mix in 1 to 2 tablespoons additional panko. If time allows, refrigerate the patties for 10 minutes to help them firm up.
Heat 1 tablespoon oil in a large nonstick skillet over medium heat. Add 4 patties, leaving space between them. Cook for 3 to 4 minutes per side, until deeply golden and heated through. Transfer to a plate and repeat with the remaining oil and patties.
Serve the warm salmon cakes with the chilled dill sauce, lemon wedges if desired, and your favorite simple sides.