Heat oven to 350°F. Coat 2- to 2 1/2-quart soufflé dish with nonstick cooking spray.
Bring large pot of lightly salted water to boiling. Add cauliflower; cook until very tender, about 15 minutes. Drain well.
Melt butter in small saucepan over medium heat. Sprinkle flour over top; whisk until combined. Cook over medium heat, whisking occasionally, 3 minutes. Whisk in heavy cream, 3/4 teaspoon salt, nutmeg and cayenne. Remove saucepan from heat.
Transfer cauliflower and cream mixture to food processor. Puree, in batches if necessary, until smooth. Add egg and pulse until smooth. Season with remaining teaspoon salt. Stir in parsley and 2 tablespoons Parmesan.
Transfer cauliflower mixture to prepared baking dish. Sprinkle top with remaining tablespoon cheese.
Bake in 350°F oven until instant-read thermometer inserted in center registers 160°F, about 30 minutes. Let stand 10 minutes before serving.