Preheat oven to 350°F.

In a large skillet heat butter or olive oil over medium-high heat; add onion and mushrooms and saute until golden and mushrooms begin to darken and dry out (about 15 minutes). Remove from heat and cool 15 minutes; season with salt and ground black pepper, if desired.

In a large bowl combine sour cream, mayonnaise, 1/3 cup cheese, and parsley; mix well. Dice chicken strips and fold into cooled mushroom-onion mixture, mix thoroughly.

Spread mixture evenly in a 1 quart casserole dish and sprinkle the top evenly with remaining cheese.

Bake 25 to 30 minutes or until completely heated through, bubbling and cheese is melted on top. Remove from heat and serve while hot.