Slice up the onion(1/4" slices) and toss it in a steel sautée pan with some butter on medium low heat. Come back to stir occasionally. If you hear sizzling, stir and reduce the heat slightly. Cook em down to a sweet brown mush and set aside.
Preheat oven to 450°F.
Pare cauliflower to small florets, reserving the core. Place florets on a baking tray with a little olive oil. Season with salt and pepper and roast for about 15 minutes until cauliflower is beginning to crisp and turn golden brown.
As the florets roast, peel and slice the core, then toss it in a pot with the milk. Simmer until cauliflower core is tender.
Slice or grate the cheese. Pour the milk and cauliflower mixture into a blender. Purée, adding the cheese slowly. Season with salt and pepper and continue blending to achieve a smooth consistency.
Spread the caramelized onions in the bottom of a baking dish. Scatter the cauliflower on top. Pour your cheese sauce evenly over the top to cover the cauliflower.
Bake, uncovered, until golden brown.
