Saute mushroom crowns gently in the 1/4 cup butter for 3 minutes, stirring carefully so caps do not break.
Remove from pan and place on a greased cooky sheet, hollow side up.
Saute onions until limp in the remaining butter; put about 1/4 teaspoon chopped onion inside each mushroom cap.
Dip oysters in melted butter; place 1 inside each mushroom; sprinkle lightly with salt.
Heat under the broiler until the edges of the oysters start to curl.