Tom Kerridge's Fish burgers with herb mayonnaise

Ingredients

• 350g skinless pollock fillet or other white fish
• 200g skinless mackerel fillet
• 100g raw prawns
• 1 tbsp sea salt flakes
• 2 garlic cloves, crushed
• 2 tbsp capers in brine, drained, rinsed and chopped
• 1 tsp chopped dill
• 1 tsp English mustard powder
• 1 tsp bicarbonate of soda (optional)
• 1 beef tomato
• ½ small onion, thinly sliced
• 4 white burger buns, halved
• 4 slices of pancetta
• 1 Little Gem lettuce, separated into leaves, rinsed and dried
• 2 tbsp chopped flat-leaf parsley leaves
• 1 tbsp chopped tarragon
• 1 large egg yolk
• 2 tsp white wine vinegar
• 1 tsp Dijon mustard
• 150-175ml vegetable oil