1Heat oven to 425°F. In each of 2 (10-oz) custard cups, place 2 teaspoons butter. Heat in oven until bubbling.
2Meanwhile, in small bowl, beat flour, milk, egg and salt with wire whisk just until smooth. Pour mixture into bubbling butter. Sprinkle rhubarb over batter. In small bowl, mix brown sugar and nutmeg; sprinkle over batter and rhubarb. Place cups in 11x7-inch glass baking dish.
3Bake 20 to 25 minutes or until puffed and golden brown. Serve warm.