Turn cauliflower upside down and cut all around the stem with sharp knife. Remove tough leafy stems. Do not cut the florets apart. Spray 8x8 baking dish with cooking spray. Rinse cauliflower sections and place without draining into baking dish. Cover dish with upside plate or piece of waxed paper. Microwave 3 minutes on high. Turn oven to 400 degrees. In large pan, melt butter over medium heat. Add onion and garlic and saute’ until fragrant, 1-3 minutes Add flour and stir to form a roux. Cook, stirring contantly about 1 minute. Lower heat if necessary to avoid browning the roux. Add cream/milk slowly, whisking to keep the mixture smooth. Bring mixture to slow boil, and then reduce heat to a simmer and cook until thick, smooth and creamy, about 5 minutes. Add nutmeg, salt and pepper and grated cheese, stirring until cheese is melted. Pour evenly over the cauliflower in the 8x8 baking dish and sprinkle grated parmesan evenly over the top. Bake for 20-30 minutes until golden brown and bubbly.