How to Make It
Preheat oven to 425°F. Bring 3 quarts water to a boil in a large pot over high. Season with 1/3 cup salt. Add cauliflower, and cook, stirring occasionally, until tender, about 8 minutes. Using a slotted spoon, transfer cauliflower to a large bowl filled with ice water. Let stand 5 minutes. Drain cauliflower, and arrange in a single layer on a baking sheet lined with paper towels; set aside.
Melt 2/3 cup butter in a large saucepan over medium. Gradually whisk in flour. Cook, whisking constantly, until mixture is smooth, about 1 minute. Gradually whisk in warm milk. Cook, whisking constantly, until mixture is thick and bubbly, about 2 minutes. Gradually whisk in cheddar, Ogleshield, Stilton, white pepper, nutmeg, and remaining 2 teaspoons salt until smooth.
Transfer cauliflower and 5 cups cheese sauce to a large bowl; stir to coat. Spoon mixture into a 2 1/2-quart baking dish. Pour remaining 1 cup cheese sauce over top. Bake in preheated oven until edges are bubbly, 15 to 20 minutes. Meanwhile, melt remaining 2 tablespoons butter. Place melted butter, panko, and Parmesan in a small bowl; stir to combine. Sprinkle panko mixture over casserole, and bake at 425°F until golden brown, 6 to 8 minutes.