Cauliflower Gratin

Bahan-bahan (6 porsi)

• 1 medium head cauliflower (about 2½ lbs/1.1kg), cut into medium florets
• 3 tablespoons unsalted butter
• 1/4 cup all-purpose flour
• 2 cups whole milk, warmed (see note)
• Salt
• Freshly ground black pepper
• Pinch of ground nutmeg
• 1 cup shredded Gruyère, divided
• 1/4 cup grated Pecorino Romano, divided
• 1/2 cup panko breadcrumbs
• 1 tablespoon extra-virgin olive oil
• 2 teaspoons fresh thyme (or ¾ teaspoon dried)