1. Heat milk, butter or margarine, 1/2 tea spoon of the onion salt, and pepper just to boiling in a medium-size saucepan. Add flour all at once; stir vigorously with a wooden spoon 2 minutes, or until batter forms a thick smooth ball that follows spoon around pan. Remove from heat.
2. Beat in eggs, 1 at a time, until batter is shiny-smooth.
3. Set aside 14 cup of the diced cheese; stir remainder into mixture in saucepan. Drop batter by heaping tablespoonfuls onto a large cooky sheet to form a rectangle about 12x5. (Batter will spread during baking to make a long loaf.) Sprinkle with the 1/4 cup cheese.
4. Bake in moderate oven (375°) 45 minutes, or until puffed and golden.
5. While puff bakes, combine chicken and celery in a large bowl. Blend mayonnaise or salad dressing, deviled ham, lemon juice and remaining 1/2 teaspoon onion salt in a small bowl; fold into chicken mixture.
6. Cut a thin slice from top of puff; scoop out any bits of soft dough from bottom with a teaspoon. Spoon chicken mixture into bottom ; set top back in place.
7. Bake 10 minutes longer, or until filling heats through. Carefully slide onto a large serving platter; cut crosswise into 4 thick slices.