Preheat the oven to 200°C, gas mark 6.
Place the sausages, onion, tomatoes, mustard, 1tbsp tomato relish and oil in a large baking tray. Mix well to coat. Bake for 40-45 minutes until cooked through, golden and caramelised, stirring once half way through.
Spread the remaining relish over the bread and fill with the sausage and onion mixture.
For top tips on protecting you and your family when preparing raw meat and poultry, visit Food Safety in the Home.