Place the kohlrabi and 1/2 teaspoon salt in a saucepan. Cover with water, and bring to a boil over medium-high heat. Cook until kohlrabi can be pierced with a fork, but remains firm, about 5 minutes. Drain, reserving 1 cup of cooking water. Place kohlrabi in a bowl, and cover.
Place the butter into the same saucepan, and melt over medium heat. Whisk in the flour, and stir until the mixture becomes paste-like and golden brown. Gradually whisk the milk and reserved cooking water from the kohlrabi into the flour mixture, stirring until thick and smooth. Stir in the cream, 1 teaspoon salt, nutmeg, white pepper, and parsley until well blended. Continue whisking until sauce thickens, then cook 10 minutes more. Stir in the kohlrabi, tossing to coat evenly with sauce.