Grease two 2-cup souffle dishes or custard cups; sprinkle each with 1/2 teaspoon of sugar. Set aside. Place sweet potato in a saucepan; cover with water. Cover and cook until tender, about 10-15 minutes; drain well. Add lemon juice, salt and nutmeg. Mash potato mixture; set aside to cool.
In another saucepan, melt butter; stir in flour and remaining sugar until smooth. Gradually add milk; cook and stir until thickened and bubbly. Remove from the heat.
Beat egg yolks in a small bowl; stir in mashed potato. Combine potato and milk mixture; set aside. In a bowl, beat egg whites until stiff peaks form; gently fold into potato mixture.
Spoon into prepared dishes. Top with marshmallows. Bake, uncovered, at 350° for 35-38 minutes or until a knife inserted in the center comes out clean. Serve immediately.