Mock Chicken Divan Pot Pie

Name: Mock Chicken Divan Pot Pie
Ingredients: 4 cups broccoli florets (halved if large); 11/4 cups prepared creamy portobello mushroom soup, such as Imagine; 3/4 cup light mayonnaise or soy mayonnaise; 1 tsp. dry sherry or cooking sherry; 1/4 tsp. poultry seasoning (or 1/8 tsp. each ground thyme and ground sage); 1 lb. seitan, cut into bite-sized pieces; 1/2 cup freshly grated Parmesan cheese or shredded vegan cheese of choice, optional; 2/3 sheet (1/3 of 17.3-oz. pkg.) frozen puff pastry, thawed