1 pound mushrooms, sliced thin
1/4 cup unsalted butter
1/4 cup flour
6 tablespoons heavy cream
2 large onions (1 3/4 pounds)
1 cup grated Gruyère cheese
1/4 cup fresh fine breadcrumbs

In a skillet cook the mushrooms in 2 1/2 tablespoons butter over
moderately low heat, stirring, until they are softened and most of
the liquid they give off has evaporated. Stir in the flour and cook
the mixture, stirring, 3 minutes.
In each of six 1 1/2-cup gratin dishes, layer the onions, the remaining
butter, cut into bits, the mushroom mixture and salt and pepper to taste,
beginning and ending with a layer of onion. Pour 1 tablespoon cream
over the top of each gratin.
In a small bowl combine well the Gruyère and the breadcrumbs and
sprinkle the mixture over the cream.
Alternatively, the gratin may be prepared, layered in the same manner,
in one 2-quart gratin dish.
Bake 50 minutes to 1 hour at 325 degrees F. or until onions are tender
and tops are golden. (Can be prepared 3 hours in advance and reheated
5 minutes at 400 degrees F.)