Mushroom Spinach and Artichoke Casserole

2 - 10 ounce packages of frozen, chopped spinach, cooked and drained
20 ounce can of artichoke hearts, drained
1/2 pound diced mushrooms
6 tablespoons of butter
1 tablespoon flour
1/2 cup milk
1 cup sour cream
3/4 cup mayonnaise
1/4 cup lemon juice
1/2 teaspoon salt
1/8 teaspoon garlic powder

Saute mushrooms in 4 tablespoons of butter. Add flour, milk, salt, garlic powder and two tablespoons melted butter. Cook until thickened. Add spinach and mushrooms. Arrange artichokes in a 9x13 inch greased pan. Pour the spinach and mushroom mixture over the top of artichokes. Blend sour cream, lemon juice and mayonnaise. Heat until smooth. Pour over top of artichokes, spinach and mushrooms. Bake 20 minutes at 375 degrees.