Grease a 9x9 dish or 2 1/2 quart dish.
Steam cauliflower florets for 5-7 minutes or until crisp tender. It will continue to cook in the oven.
While cauliflower is steaming, make the sauce. In a saucepan, melt the butter. Add the onions and cook until tender, adding the garlic for the last minute of cooking. Sprinkle the flour over the butter and cook for a minute. Whisk in the cream, salt, and pepper. Cook until mixture thickens. Remove from the heat and stir in 1 cup of the cheddar cheese until melted.
Pour over the cauliflower. Top with remaining 1/2 cup of cheese. Cheese will caramelize while cooking. If you like a softer cheese top, wait to add cheese the last 5 minutes of cooking. Bake for 20 minutes at 350 degrees.