01 - Preheat your oven to 400°F (200°C). Lightly grease a 9x13-inch baking dish to prevent sticking.
02 - Bring a large pot of salted water to a rolling boil. Add the cauliflower florets and cook for 5–6 minutes, until they are just tender but still hold their shape. Drain the cauliflower thoroughly and set aside.
03 - In a medium saucepan, melt 2 tablespoons of butter over medium heat. Whisk in the gluten-free all-purpose flour and cook for 1 minute, stirring constantly, to create a smooth paste.
04 - Gradually whisk in the whole milk, ensuring no lumps form. Add the Dijon mustard, garlic powder, onion powder, black pepper, and salt. Continue to cook, whisking constantly, until the sauce thickens to a creamy consistency, approximately 4–5 minutes.
05 - Remove the saucepan from the heat. Stir in the shredded sharp cheddar cheese and grated Parmesan cheese until the mixture is smooth and fully melted, creating a rich cheese sauce.
06 - Gently add the drained blanched cauliflower to the cheese sauce. Toss carefully to coat the cauliflower florets evenly with the sauce.
07 - Pour the cheesy cauliflower mixture into the prepared baking dish, spreading it out evenly.
08 - If using the topping, in a small bowl, combine the gluten-free breadcrumbs, melted butter, and grated Parmesan cheese. Sprinkle this mixture evenly over the top of the cauliflower casserole.
09 - Bake for 20 minutes, or until the topping is golden brown and the casserole is bubbling around the edges. For an extra crisp topping, you may broil for an additional 2–3 minutes, watching carefully to prevent burning.
10 - If desired, sprinkle with fresh chopped chives or parsley before serving. Serve hot as a main dish or a flavorful side.