Spinach can be added in batches if your skillet does not accommodate all of the spinach at once. As soon as the spinach begins to wilt, add the remaining spinach.
Substitute 4 pkg. (10 oz. each) frozen spinach leaves, thawed and drained, for the fresh spinach. Cook mushrooms and garlic as directed. Add spinach; cook until heated through, stirring frequently. Continue as directed.