Preparation

• Add the cauliflower to a medium pot, then add just enough room temperature water to barely cover.
• Bring the water up to a simmer, then cover and cook until the cauliflower pieces are al dente, about 2-3 minutes. Drain the cauliflower well and reserve the cooking water.
• Preheat the oven to 400℉ and butter the inside of a medium casserole dish.
• Melt butter in a medium sauce pan over medium heat, then whisk in the flour until it absorbs all the butter, about 1-2 minutes.
• Slowly whisk in 1½ cups of the reserved cauliflower cooking water (it should still be warm), then continue whisking until it comes up to a boil.
• Lower the heat, continue whisking, and cook for 2-3 minutes until thickened.
• Whisk in the heavy cream, salt and pepper, then turn off the heat.
• Stir in the nutmeg and ⅓ of the Gruyere cheese (about 1 cup) until the cheese is melted.
• Add the well-drained cauliflower to the cheese sauce and mix to combine. Taste for seasoning and adjust as needed.
• Scrape the cauliflower mixture into the buttered casserole dish, being sure to get every last bit of cheese sauce in there.
• Sprinkle with the remaining Gruyere cheese, then set aside while you make the topping.
• Melt the butter in a medium sauté pan over medium heat, then add the garlic and stir around until it starts to sizzle.
• Add the panko breadcrumbs, thyme leaves and a pinch of salt and pepper, then continue to stir around until light golden brown, about 5 minutes.
• Spread the breadcrumbs on top of the cauliflower in an even layer.
• Bake for about 20 minutes, or until the breadcrumbs have darkened a few shades and it's bubbling up around the sides.
• Allow the gratin to cool down for about 10 minutes before serving.