Cook spinach according to package directions; drain well.
Place chicken in saucepan with celery, onion, salt, and 1 cup water.
Bring to boil; reduce heat and simmer till meat is tender, about 20 minutes.
Remove chicken from broth.
Strain broth; reserve 1 cup.
Discard vegetables.
In saucepan melt butter; blend in flour and pepper.
Stir in reserved broth and cream.
Cook and stir till thickened and bubbly.
Remove from heat; stir 1/2 cup of the sauce into drained spinach along with half the cheese and the nutmeg; spread in a 10 x 6 x 2 inch baking dish.
Arrange chicken atop.
Pour remaining sauce over all.
Sprinkle with remaining cheese and more nutmeg, if desired.
Bake, uncovered, at 375° till lightly browned, 25 to 30 minutes.