Drain salmon, reserving liquid; add enough milk to salmon liquid to measure 1 cup set aside.
Melt butter in a heavy saucepan over low heat. Add onion and cook until tender. Add flour, stirring until smooth. Cook over medium heat stirring constantly until thick and bubbly. Stir in salt and pepper; set aside.
Remove skin and bones from salmon, flake with fork. Add lemon juice, 1/2 cup corn flakes and white sauce, stirring well. Shape into croquettes. Roll in remaining corn flakes. Place in lightly greased baking sheet. Bake at 400 degrees for 30 minutes.
