In a small skillet, saute mushrooms in butter until tender; set aside. In a large bowl, beat the cream cheese, eggs, milk and biscuit mix just until combined. Stir in the broccoli, cheeses, salt and mushrooms.
Pour into a greased round 2-1/2-qt. baking dish.
Bake, uncovered, at 350° for 50-60 minutes or until a thermometer reads 160°.