Cut meat lengthwise about two-thirds through.
Arrange prunes in the "pocket" and fasten with skewers.
Brown meat on all sides in heated butter in a large skillet.
Sprinkle with 1 teaspoon salt.
Cover and cook slowly about 1 1/2 hours, or until meat is tender.
During cooking, add a small amount of water.
Remove meat to a hot platter, remove skewers and keep hot.
Leaving brown residue in skillet, pour drippings into a bowl.
Allow fat to rise to surface; skim off fat, put 3 tablespoons of the fat into skillet, and reserve 1/2 cup of the drippings.
Blend in flour, 1/4 teaspoon salt, and pepper.
Heat until bubbly, stirring constantly.
Stir in milk and reserved drippings.
Cook and stir 1 to 2 minutes.