Preheat a smoker to 250°F (121°C), using hickory, apple, cherry, or pecan wood. These woods provide a pleasant smoke flavor without overpowering the onion.

Peel the onions and cut them crosswise into rings about 1/2 inch thick. Separate the rings carefully, then pair a smaller ring inside a larger ring so a gap remains between them.

Combine the brown sugar, smoked paprika, garlic powder, black pepper, and optional cayenne pepper in a small bowl. Set the seasoning mixture aside while you prepare the bacon.

Wrap each paired onion ring with bacon, slightly overlapping the slices as you work around the ring. Most large rings need 1 1/2–2 bacon slices; secure the ends with toothpicks.

Arrange the wrapped rings on a wire rack, perforated grill pan, or smoker grate with space between them. Smoke for 60–90 minutes, until the bacon has rendered some fat and the onion is beginning to soften.

Brush the rings generously with barbecue sauce during the latter part of cooking. Continue smoking for another 30–45 minutes, until the onion is tender and the bacon is cooked through.

Transfer the rings to a hot grill, a 400°F oven, or a broiler for 5–10 minutes, turning once if needed. Watch continuously because the barbecue sauce can darken or burn quickly at high heat.

Rest the smoked onion rings for 5 minutes. Remove the toothpicks carefully and serve warm with the remaining barbecue sauce.