Put the butter into a 9-inch round casserole pan or oven-proof dish of a similar size and place over medium heat. Add the onion and saute for 8 minutes, until soft and golden. Add the cumin, curry powder, mustard powder, and chiles and cool for 4 minutes, stirring occasionally. Add the mustard seeds, cook for 1 minute, then pour in the cream. Add 1 ¼ cups of cheddar and ½ tsp of salt and simmer for 2-3 minutes, until the sauce slightly thickens. Add the cauliflower, stir gently, and simmer for 1 minute before removing from the heat.