No Spanish tapas table would be complete without some form of crispy croquettes.
In a heavy saucepan, combine butter and flour. Stir with a whisk or wooden spoon until smooth and bubbly. Slowly whisk in milk and cook, stirring constantly, until mixture thickens. Add black pepper.
In a bowl, combine parsley, celery, green onion, mustard, 1 egg, and ham. Toss until blended. Stir ham mixture into thickened white sauce. Pour mixture into a shallow dish. Cover and refrigerate for 45 minutes, or until mixture can be shaped.
Whisk together remaining egg with a small amount of water. Pour bread crumbs onto a plate. With a small spoon, scoop up ham mixture and shape into 2-inch oblong balls. Gently dip into egg, then coat with bread crumbs. Set aside on a platter and continue forming croquettes. Mixture should yield 32–36 croquettes.
Heat about 2 inches of vegetable oil in a heavy frying pan. When oil is hot, slip croquettes in. Turn frequently and cook until browned on all sides. Drain on paper towel and keep warm until ready to serve.