Preheat the oven to 350 degrees F (175 degrees C). Lightly grease a small baking dish.
Place the cauliflower in a saucepan with enough water to cover. Bring to a boil and cook 5 minutes, or until tender but crisp.
In a separate saucepan, melt the butter over medium heat and gradually mix in the flour, salt, and milk. Bring to a boil, remove from heat, and mix in the Swiss cheese until melted. Spread about 1/4 cup of the melted Swiss cheese mixture in the bottom of the prepared baking dish. Place the cauliflower in the dish, and top with the remaining cheese mixture.
Bake uncovered 20 to 25 minutes in the preheated oven, until bubbly.