Pickled Mackerel with Potato and Cress Salad

Ingredients

• 8 mackerel fillets (about 150gm each), pin-boned
• 500 ml white wine vinegar, or enough to cover (2 cups)
• 2 golden shallots, coarsely chopped
• 2 garlic cloves, crushed
• 1 fresh bay leaf
• 5 black peppercorns
• 1 litre olive oil, or enough to cover (4 cups)
• Dutch cream potato and cress salad
• 1 1/2 kg Dutch cream potatoes (about 10), scrubbed
• 60 gm mayonnaise (¼ cup)
• 1½ tbsp salted capers, rinsed and drained
• 4 golden shallots, finely chopped
• ½ cup flat-leaf parsley, coarsely chopped (firmly packed)
• ¼ cup finely chopped chives
• 1 punnet mustard cress, trimmed