Heat oven to 375° F. Butter a shallow 2-quart baking dish. Fill a large pot with 1 inch of water and fit with a steamer basket; bring the water to a boil. Place the cauliflower in the basket, cover, and steam until very tender, 12 to 15 minutes; drain well.Meanwhile, melt the butter in a large saucepan over medium heat. Sprinkle with the flour and cook, stirring, for 2 minutes. Gradually whisk in the milk and bring to a boil. Reduce heat and simmer, stirring occasionally, until thickened, 6 to 8 minutes.Whisk in the cheese, nutmeg, 1 ½ teaspoons salt, and ¼ teaspoon pepper and cook, whisking, until the cheese is melted, 1 to 2 minutes. Remove from heat and gently fold in the cauliflower; transfer to the baking dish.In a bowl, toss together the bread crumbs, parsley, oil, and ¼ teaspoon each salt and pepper. Sprinkle over the cauliflower and bake until golden brown and bubbling, 15 to 20 minutes.