1Preheat oven to 200°C. Place cauliflower florets, chopped stalk, and leaves on a baking tray. Drizzle with olive oil and season with salt and pepper. Roast for 20–25 minutes, turning once, until golden and slightly charred.
2Melt butter in a saucepan over low heat. Add flour and whisk for 2–3 minutes. Gradually pour in ale, whisking constantly. Add milk and whisk until sauce thickens.
3Stir in grated cheddar cheese until smooth. Add Dijon mustard, nutmeg, salt, and pepper. Simmer gently for a few minutes.
4Place roasted cauliflower in an ovenproof baking dish. Pour the cheese sauce over the cauliflower.
5Top with grated cheddar, Gruyère, and Parmesan cheeses.
6Sprinkle with panko breadcrumbs and cracked black pepper.
7Bake for 20–25 minutes until the top is bubbling, golden, and crispy.