In a large skillet, melt butter over medium-high heat. Add onion; cook, stirring frequently, until lightly browned, about 15 minutes. Reduce heat to medium-low; cook until golden brown, about 20 minutes more. Stir in cream, thyme, salt, pepper, and nutmeg, and bring to a low boil.
In a 2-quart baking dish, place an even layer of sweet potato slices. Drizzle with 3 tablespoons cream mixture. Repeat layers to use all sweet potatoes and cream mixture. Cover with foil.
Bake for 30 minutes. Uncover and bake until potatoes are tender and top is golden brown, about 1 hour more, sprinkling with Parmesan during last 10 minutes of baking. Garnish with thyme, if desired. Let stand for 15 minutes before serving.
Notes
Kitchen Tip: A mandoline is the best tool for getting thin slices of sweet potatoes.
Recipe by Paula Deen Magazine at https://pauladeenmagazine.com/caramelized-onion-scalloped-sweet-potatoes-recipe/