Steam the cauliflower until barely done. If you don't have a steamer basket, I would microwave in a covered dish with a small bit of water.
Preheat the oven to 350.
Put the cauliflower into a small buttered baking dish.
Melt the butter in a small saucepan and then stir in the flour. Cook over medium-low heat 2 minutes.
Heat the milk in a microwave during the two minutes if you'd like. It makes it easier to blend.
Slowly whisk the milk into the butter and flour, then bring to a boil. Turn down the heat and simmer a few minutes, stirring, until it has thickened.
Take the pan off the heat and stir in 1/2 cup of cheese.
Pour the cheese sauce over the cauliflower.
Top with a bit more grated cheese if you'd like.
Bake uncovered for 20-25 minutes.