Preheat the oven to 350°. Put the sweet potatoes on a baking sheet and roast for 40 minutes, or until tender.
Meanwhile, heat the olive oil in a medium skillet. Add the red onion, cover and cook over low heat, stirring a few times, until caramelized, about 15 minutes. Remove from the heat and stir in the vinegar; season with salt and pepper.
In a small bowl, mix the yogurt with the garlic and season with salt and pepper.
Split the sweet potatoes, leaving them attached at the bottom. Season the insides with salt and pepper. Spoon 1 tablespoon of the garlicky yogurt into each potato, then fill with the caramelized onion. Serve, or gently close the sweet potatoes and wrap them in foil for packing.
Make Ahead: The potatoes can be wrapped for up to 4 hours.
Notes: One Serving: 232 calories, 2 gm total fat, 7 gm saturated fat, 46 gm carb.