Preheat oven to 450°F.
In a large bowl, combine the flour, onion salt and celery salt.
Combine the eggs, milk, onion, celery and butter; whisk into the dry ingredients just until blended.
Grease and flour the bottom and sides of 12 popover cups; fill two-thirds full with batter.
Bake at 450°F for 15 minutes.
Reduce heat to 350°F (do not open oven door) and bake 25 minutes longer or until deep golden brown (do not underbake).
Immediately cut a slit in the top of each popover to allow steam to escape.