BBC Food - Recipes - Fresh sardines on toast with sweet pickled red onion

Ingredients

• 4 whole sardines , cleaned, heads removed
• 1 tbsp plain flour
• flaked sea salt
• freshly ground black pepper
• 3 tbsp olive oil
• 1 red onion , sliced
• 1 tbsp caster sugar
• 2 tbsp red wine vinegar
• 1 tbsp small capers , drained
• 1 heaped tbsp chopped fresh parsley
• 4 slices sourdough bread
• 50g/1¾oz butter , softened
• 2 tsp wholegrain mustard
• 1 ripe tomato , finely sliced