This is the Julia Child version, so don't faint over the amount of dairy involved. :-)
From frozen spinach:
Thaw two 10-oz packages of frozen spinach just until you can cut through it with a heavy knife. If spinach is chopped, slice into 1/2" pieces. If whole, chop into small bits.
From fresh spinach:
Blanch 1.5 lbs. spinach in salted boiling water until wilted and almost tender. Strain, squeeze out liquid, and chop.
Melt 1 1/2 T butter in a heavy saucepan
For frozen: Cook, covered, over low heat for a couple minutes until for spinach has thawed and released its juices.
For both: Cook over moderately high heat, uncovered, 2-3 minutes until moisture is evaporated.
Add 1/4 t salt, pinch of pepper, small pinch of nutmeg.
Lower heat to moderate and sprinkle on 1 T flour, sifted. Cook, stirring, for 2 minutes
Remove pan from heat and slowly add 1/2 cup heavy cream by spoonful, stirring to incorporate each spoonful as it is added. Bring to simmer, cover, and cook on low about 15 minutes, stirring frequently to keep spinach from sticking to pan and adding additional spoonsful of cream as necessary if spinach becomes too dry.
Remove from heat and fold in 1 T softened butter. Serve.