Cut sweet potatoes in half lengthwise and then crosswise into 1/2-inch half-moons. Drizzle with olive oil on a baking sheet and turn to coat. Roast 40-50 minutes until all potatoes are soft. Salt and pepper them lightly.
Simmer cider vinegar, mustard seeds and water until seeds are plump and most of the liquid has absorbed or evaporated, about 20 minutes. Drain and reserve any excess liquid.
Fry bacon in a skillet until fat begins to separate, about 4 minutes. Add onion and cook until bacon and onion are crisp and browned. Add mustard seeds and cook about 1 minute. Remove from heat and add 2 tablespoons maple syrup and any reserved vinegar mixture. Mix bacon mixture into sweet potato mixture. Drizzle 1 tablespoon maple syrup on top.