Spinach Artichoke Casserole

1 onion, chopped
1/4 cup (1/2 stick) butter
8 ounces fresh mushrooms, sliced
2 packages frozen spinach
8 ounces sour cream
1 (14 ounce) can artichoke hearts, quartered
1/2 cup Parmesan

Saut onion in butter. Add mushrooms and saut until wilted. Squeeze the water out of thawed spinach.

Mix spinach, onion and mushrooms, sour cream, and artichoke hearts in a casserole dish. Top with Parmesan cheese and bake at 350 degrees F for 30 to 60 minutes (until warmed through and cheese is melted).