Preheat oven to 350°. Grease a 6-cup souffle dish with 1 tsp. butter, dust with flour (tap out excess), and set aside.
Boil turnips in a pot of salted water over high heat until soft, 8-10 minutes; drain well and mash until smooth. Melt remaining butter in a medium pan over medium heat. Add flour and cook, stirring, for 2 minutes. Stir in cream and mashed turnips and cook until thick, about 5 minutes.
Add onions and salt and freshly ground white pepper to taste. Remove pot from heat and gradually stir in egg yolks. Transfer mixture to a large bowl and set aside.
Whisk egg whites in a large bowl until stiff peaks form, gently fold into turnip mixture, and spoon into prepared dish. Bake until puffed and golden, 35-40 minutes.