Easy Baked Cauliflower Gratin

For the Filling:

• 1 large head of cauliflower trimmed and cut into florets (about 2 ½ lbs or 6-7 heaping cups)*
• 3 tablespoons butter plus more for greasing the dish
• ¼ cup all purpose flour
• 1 ½ cups reserved cooking water
• ¼ cup heavy cream
• 2 teaspoons kosher salt plus more to taste
• Freshly ground pepper to taste
• ¼ teaspoon fresh nutmeg
• 6 ounces gruyere cheese grated (about 3 cups), divided

For the Topping:

• 3 tablespoons butter
• 1 clove garlic finely minced
• 1 ¼ cups panko breadcrumbs
• 1 teaspoon fresh thyme leaves
• salt and pepper to taste