Drain spinach well, pressing between layers of paper towels. Stir together spinach and next 7 ingredients.
Melt butter in a skillet over medium-high heat. Add mushrooms and next 3 ingredients; saute 5 minutes. Stir mushroom mixture and 1 cup cheese into spinach mixture; spoon into a lightly greased 11- x 7-inch baking dish. Sprinkle with remaining 1 cup cheese.
Bake at 400° for 30 minutes.