Wash the clam shells thoroughly with a brush and clear water.
Put them into a pot on the fire with one half cup of boiling water; cover and let steam until the shells open; take out the clams and let the liquor settle; then strain it carefully, and set aside; remove the clams from the shells; chop them, pound them in a mortar, and press as much of them as possible through a puree sieve. Put the milk into a double boiler with the slice of onion. Put the butter into a frying-pan, and when it bubbles, stir into it the flour, and let it cook a few minutes, but not brown; add enough of the milk slowly to make the roux liquid; then add it to the milk in the double boiler, first having removed the slice of onion; add a dash of nutmeg and of pepper, then the cream; when ready to serve, stir in the clam pulp and one pint of the clam liquor; taste to see if salt will be needed. After the clams are added to the milk, leave it on the fire only long enough to get well heated; if boiled, the milk will curdle. Beat a moment with an egg-whisk to make foamy. If the mixture is too thick, it may be diluted with milk or cream.
This is good for luncheon, served in small cups, the top covered with a spoonful of whipped cream.