Preheat oven to 350 degrees. Cook cauliflower in a large pot of salted water. The cauliflower should be tender, but firm. The cooking time should be 5 to 6 minutes.
While the cauliflower is cooking, start the cheese sauce. Melt two tablespoons of butter in a small saucepan. When the butter has melted completely add the flour. Cook flour for 1 to 2 minutes. Slowly begin to add milk, which each addition of milk the sauce should thicken. When all of the milk is added add 4 ounces of cheese. Stir in the cheese until it melts completely.
Place cooked cauliflower into a large casserole dish. Pour cheese sauce over the cauliflower. Sprinkle remaining 4 ounces of cheese over the cauliflower. Place the cauliflower into the oven and bake for another 15 minutes. When the cheese should begin to brown slightly the gratin is done.