Add the cauliflower to a medium pot, then add just enough room temperature water to barely cover. The goal is to use less water in order to achieve a more concentrated cauliflower flavor, so it's okay if some pieces are sticking out of the top.
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 degrees F 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 1 1/2 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 1/3 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 - it will likely need a bit more salt and pepper.
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.
Notes
*Use about 2 1/2 lbs or 7 cups of frozen cauliflower.
Be careful not to overcook the cauliflower while boiling, it should still be slightly firm or al dente. The time will depend on the size of your florets - check them every minute or so with a fork.
It's really important that the cauliflower is drained very well and does not have a lot of extra liquid before adding to the cheese sauce or it will be watery.
Use a deep dish pie plate or cast iron skillet in place of a casserole dish if desired.
This cauliflower gratin can be made entirely in advance. Reheat in a 350 degree F oven covered for about 15 minutes, then remove the cover and bake for another 10-15 minutes until bubbling around the sides.