Select slender potatoes that will make perfect scallops when sliced crosswise.
In a saucepan, cover the sweet potatoes with water and boil over high heat until just tender, about 12 minutes. Drain the potatoes and let cool. Peel and slice them crosswise 1 inch thick.
In a medium skillet, heat 1 tablespoon of the oil. Add half of the sweet potato slices and cook over moderately high heat until well browned, about 3 minutes per side. Transfer the sweet potatoes to a plate and season with salt and pepper. Repeat with the remaining 1 tablespoon of oil and sweet potatoes.
Add the onion to the skillet, cover and cook over low heat, stirring occasionally, until softened and browned, about 8 minutes. Season with salt and pepper, scatter over the sweet potatoes and serve.
Wine Recommendation: This wine-friendly side dish could be happily paired with a nonvintage sparkling wine, such as the Zardetto Prosecco from Italy. The onions make it an especially good match with the fruity 1996 or 1997 Georges Duboeuf Beaujolais.