Preheat your oven to 200°C (390°F). Lightly butter your baking dish to prevent sticking.
Trim the cauliflower into small, even florets and bring a large pot of salted water to a boil. Blanch the cauliflower for 3-4 minutes until just tender but still firm. Drain well and set aside.
In a saucepan, melt the butter over medium heat, then add the flour, whisking constantly for about 1 minute until the mixture is smooth and slightly bubbly. This creates a roux for your béchamel.
Gradually pour in the milk while whisking continuously to prevent lumps. Bring the mixture to a gentle simmer and cook for 3-4 minutes until it thickens into a smooth, glossy sauce.
Stir in the grated cheddar cheese, Dijon mustard, a pinch of nutmeg, salt, and black pepper. Keep whisking until the cheese melts completely and the sauce is rich and velvety.
Add the drained cauliflower to the cheese sauce, gently folding to coat each piece evenly. Transfer this mixture into your prepared baking dish.
Mix the breadcrumbs with a teaspoon of melted butter and dried thyme, then sprinkle evenly over the top of the cauliflower mixture. This will give a golden, crunchy topping.
Bake in the preheated oven for 20-25 minutes until the top is golden brown and bubbling around the edges. The aroma of melted cheese and toasted breadcrumbs will fill your kitchen.
Remove from the oven, let the bake rest for 5 minutes to set. This makes slicing easier and enhances the flavors.
Serve hot, garnished with a sprinkle of fresh herbs or a squeeze of lemon if desired. Enjoy the creamy, cheesy goodness with a crispy topping.
