Saute mushrooms in half the butter. Add onions and saute until tender. Add cream cheese and stir until melted; stir in spinach and garlic salt. Using 3 sheets filo dough, spread on flat surface, brush with melted butter and spoon some filling along one long side of dough, then roll up jelly roll fashion. Place in lightly buttered jelly roll pan. Brush top with melted butter. Repeat to use all filling. Cover and refrigerate until firm. Bake at 400 degrees for 25 minutes. Cut into 1 to 1 1/2 inch pieces.