Retro Mushroom Casserole

I love old cookbooks. I found this recipe for Mushroom Casserole in the 1970 edition of the Junior League of Peoria, Illinois "Marigolds to Munch On" cookbook.  Whoever had owned this book before me had noted this recipe as delicious!  I bet it is!

Retro Mushroom Casserole

1/3 cup butter, softened
1 Tbsp snipped parsley
1 Tbsp prepared mustard
1 Tbsp grated onion
1/8 tsp cayenne pepper
1 tsp salt
2 1/2 Tbsp flour
1/8 tsp nutmeg
1 cup heavy cream
1 lb washed fresh mushrooms

Preheat oven to 350.  Blend together all ingredients except mushrooms.  Slice the mushrooms in half lengthwise.  Place in the bottom of a 1 1/2 quart greased casserole.  Dot with half of the butter mixture.  Top with rest of the mushrooms, then rest of the butter mixture.  Pour 1 cup heavy cream over all; then bake uncovered for about 55 minutes.