Drizzle a jelly-roll pan evenly with oil, tilting pan to coat.
Combine grated onion and potatoes in a sieve; squeeze out excess moisture. Lightly spoon flour into a dry measuring cup; level with a knife. Combine potato mixture, flour, and next 5 ingredients (through egg) in a large bowl. Divide mixture into 8 equal portions, squeezing out excess liquid. Shape each portion into a 1/4-inch-thick patty; place on prepared pan. Lightly coat tops of patties with cooking spray. Bake at 425° for 12 minutes. Carefully turn patties over; cook 30 minutes or until lightly browned, turning every 10 minutes. Garnish with green onion strips, if desired.