Preheat oven to 450 degrees. Place grated onion in a fine sieve, and press out as much moisture as possible. Combine onion, potatoes, chopped dill, salt, pepper, and egg white in a medium bowl, and stir to combine.
Coat two baking sheets with olive-oil cooking spray. Form pancakes by dropping about 1 tablespoon mixture on tray for each and flattening into disks. Bake until golden brown on bottom, about 10 minutes, rotating trays once during cooking. Remove from oven, and turn pancakes over. Return to oven, and bake until brown on both sides, about 7 minutes more.
Serve pancakes topped with gravlax and a dab of reduced-fat sour cream. Garnish with dill sprigs.