Trim and discard stems from kale. Thoroughly wash and drain. Cut kale into 1/2-inch wide ribbons; set aside.
For sauce, in a medium skillet, melt butter over medium heat. Add onion and cook about 5 minutes or until tender. Stir in flour. Add milk all at once; cook and stir until thickened and bubbly. Cook and stir for 1 minute more. Stir in cheese, nutmeg, and crushed red pepper, if desired. Keep warm.
Meanwhile, bring an 8-quart Dutch oven or kettle of lightly salted water to boiling. Gradually add kale to boiling water; cook for 5 minutes or until tender. Drain well. Return to Dutch oven or kettle. Stir sauce into cooked kale. Heat through. Top with additional Parmesan and crushed red pepper if desired.