Maple-glazed hot dogs with mustardy onions

INGREDIENTS

• 8 good-quality pork sausages
• 2 tbsp sunflower oil
• 2 onions , thinly sliced
• 1 tsp yellow or black mustard seeds
• 2 tbsp maple syrup
• 4 part-baked mini baguettes (or cut a French stick into 4 pieces)
• 1 tbsp Dijon mustard
• large pinch brown or muscovado sugar
• 2 tsp wine or cider vinegar