Preheat the oven to 375°F (190°C).
Steam the cauliflower florets until they are tender, about 10 minutes. Drain and set aside.
In a saucepan over medium heat, melt the butter and add the flour, stirring to form a roux. Cook for 1-2 minutes without letting it brown.
Gradually whisk in the milk, and cook until the sauce thickens. Add the salt and pepper.
Add 1 cup of shredded cheddar cheese to the sauce and stir until melted and smooth.
Place the cauliflower florets in a baking dish, pour the cheese sauce over them, and sprinkle the remaining cheddar cheese on top.
Bake in the preheated oven for 20 minutes, or until the cheese is golden and bubbly.