Perfect Roast Pork With Crisp Crackling

Ingredients

• 3
  pounds pork loin (preferably free-range)
• 4
  tablespoons extra-virgin rapeseed oil (or olive oil)
• Sea salt flakes
• 1
  medium onion (halved with skin on)
• 2
  teaspoons flour
• 1
  cup dry cider (or white wine)
• 2
  cups chicken (or vegetable stock)
• 1
  teaspoon butter (ice-cold)