Add the cauliflower florets to a medium pot and cover with just enough room temperature water to barely submerge them. Bring to a simmer, cover, and cook until the cauliflower is al dente, about 2-3 minutes. Drain well, reserving the cooking water.

Preheat your oven to 400°F (205°C). Butter the inside of a medium casserole dish to prevent sticking.

In a medium saucepan over medium heat, melt 3 tablespoons of butter. Whisk in the flour and cook until it absorbs the butter and slightly toasts, about 1-2 minutes.

Slowly whisk in 1½ cups of the reserved cauliflower cooking water (warm). Bring to a boil, then reduce heat and whisk continuously until the sauce thickens, about 2-3 minutes.

Whisk in the heavy cream, 2 teaspoons salt, freshly ground pepper, and nutmeg. Turn off heat and stir in one-third of the grated Gruyere cheese (about 1 cup) until melted and smooth.

Add the drained cauliflower to the cheese sauce, mixing gently to coat evenly. Taste and adjust seasoning with additional salt and pepper if needed.

Transfer the cauliflower mixture into the buttered casserole dish, scraping to include all the cheese sauce. Sprinkle the remaining Gruyere cheese evenly over the top.

In a medium sauté pan, melt 3 tablespoons butter over medium heat. Add minced garlic and cook until fragrant and sizzling. Stir in panko breadcrumbs, fresh thyme leaves, and a pinch of salt and pepper. Continue cooking and stirring until the breadcrumbs are golden brown, about 5 minutes.

Evenly spread the golden breadcrumb mixture over the cauliflower. Bake in the preheated oven for about 20 minutes, or until the topping is browned and the gratin is bubbling around the edges.

Remove from oven and let the gratin cool for about 10 minutes before serving to set and enhance flavors.