Prepare the Cauliflower: Preheat your oven to 375°F (190°C). In a spacious saucepan, heat water with a pinch of salt until it reaches a rolling boil. Add the cauliflower florets and cook for about 5 minutes, just until tender but still firm. Drain and set aside.

Make the Cheese Sauce: In a medium-sized saucepan, gently warm the butter over moderate heat until it is fully melted. Once melted, whisk in the flour to form a roux, stirring constantly for about 2 minutes to cook out the raw flour taste. Gradually add the milk, whisking continuously to avoid lumps, and bring the mixture to a simmer.

Season the Sauce: Add the sprig of fresh thyme, black pepper, nutmeg, and a pinch of kosher salt to the sauce. Cook, stirring frequently, for 5-7 minutes until the sauce thickens. Remove from heat, discard the thyme sprig, and stir in the Gruyère and Parmesan cheeses until melted and smooth.

Assemble the Gratin: In a buttered 9x13-inch baking dish, spread the cauliflower florets in an even layer Drizzle the cheese sauce over the cauliflower, making sure each floret is thoroughly covered. Sprinkle the fresh breadcrumbs evenly over the top for a crispy finish.

Bake the Gratin: Transfer the casserole to the preheated oven and let it bake for 25 to 30 minutes, or until the surface is a rich golden brown and bubbling delightfully. If you like an extra crispy topping, broil for an additional 2-3 minutes, watching carefully to prevent burning.

Garnish and Serve: Take the gratin out of the oven and allow it to sit for a few moments. Top with a sprinkle of fresh thyme leaves before serving it warm.