Pull out some of the bread from the top half of each ciabatta roll; tear into small pieces and place in a medium bowl. Drizzle with 2 tbsp. water to moisten. Mix in the beef, 4 tbsp. hoisin sauce and the onion. Form into 16 golf ball/size balls.
In a medium nonstick skillet, heat 1 tsp. oil over medium-high. Add the Brussels sprouts and cook, stirring occasionally, until wilted and lightly browned in spots, 5 to 6 minutes. Stir in the honey and vinegar. Season with salt and pepper and remove from the heat.
In a large nonstick skillet, heat the remaining 2 tsp. oil over medium. Add the meatballs and cook, turning every 2 minutes, until browned all over and cooked through, about 10 minutes. Add the remaining 2 tbsp. hoisin. Cook, turning the meatballs to coat, about 30 seconds.
Divide the roll bottoms among 4 plates. Top with the Brussels sprouts mixture, the meatballs, any sauce left in the skillet and the roll tops.