2 large onions, approx. 400g/14oz, thinly sliced
1.25kg/2lb 12oz potatoes, peeled and cut into narrow strips like French fries
black pepper
2–3 tbsp breadcrumbs (optional)
2 tsp sea salt flakes, plus ½ tsp (or 1 tsp fine sea salt, plus ¼ tsp)
60²µ/2¼´Ç³ú butter, plus extra for greasing
300ml/½ pint double cream
300ml/½ pint full-fat milk
2 x 125g tins Swedish ansjovis (sprats), drained, see recipe tip