Prepare your oven and dish.
Preheat the oven to 425°F (220°C) and place an oven rack in the bottom third. Lightly coat a 13- × 9-inch baking dish with cooking spray.
Steam the broccoli.
Place the broccoli florets and 3 tablespoons (45 ml / 1.5 fl oz) of water in a microwave-safe dish with a lid. Microwave on high for 2 minutes, stir, and microwave for another 2 minutes, or until tender.
Drain well.
Mix the creamy filling.
In a large bowl, whisk together the portobello soup, mayonnaise, sherry, and poultry seasoning.
Season with salt and pepper to taste. Gently fold in the cooked broccoli and seitan pieces until everything is well-coated.
Assemble the casserole.
Pour the seitan and broccoli mixture into your prepared baking dish. If using, sprinkle the Parmesan or vegan cheese evenly over the top.
Top with puff pastry.
Cut the thawed puff pastry sheet into 6 equal squares and arrange them over the filling. It's okay if they don't cover the entire surface.
Bake to perfection.
Bake for 20 minutes, or until the pastry is beautifully puffed and golden brown, and the filling is hot and bubbling.
Let it rest for a few minutes before serving.
Don't be afraid to swap your veggies! This dish is also delicious with steamed asparagus, green beans, or even cauliflower florets.