Bring a medium pot of water to a boil. Add ½ teaspoon salt. Cook cauliflower in salted water for 3 minutes. Drain into a metal strainer and transfer to a 9-inch casserole dish.
Preheat oven to 375°F/190°C.
Melt butter in a medium saucepan. Add all-purpose flour and stir for about 3 minutes.
Pour in milk, stirring constantly. When milk comes to a boil, add remaining ½ teaspoon salt, ground pepper, and ground nutmeg.
Also add ½ cup (56g) shredded cheddar cheese. Stir until cheese is melted and sauce is thick and bubbly.
Pour sauce over cooked cauliflower. Sprinkle with remaining ½ cup (56g) shredded cheese and panko bread crumbs.
Transfer casserole to the oven and bake for 15 minutes. Remove casserole from oven and sprinkle with parmesan cheese. Return casserole to oven and continue to baked for another 15 minutes or until top is golden brown.