Hello Michelemarie
I am just guessing with the following.
I would leave the mushrooms whole, put them in the caserole dish.
Melt some butter, in a pot.
Stir in a little white flour and a sprinkling of salt.
Stirr in a cup of milk.
Heat gently and stirr constantly until sauce thickens.
Take of heat, and stirr in a handful of grated cheddar cheese.
For extra flavor, stir in a teaspoon of sugar and a teaspoon of vinegar.
Stirr in some dried herbs if u have them. Or serve with fresh ones, on the side, when serving.
Add the sauce to the mushrooms.
Put in oven, until u consider it cooked.
Gently toast the bread crumbs, under the grill.
When u take the casserole out of the oven, sprinkle the bread crumbs over it. Serve with freshly grated black pepper.
Mel