Cullen Skink, (Traditional Scottish Fish Chowder)

Cullen Skink (Traditional Scottish Fish Chowder) 4 cups 950ml whole milk1 bay leaf1 pound 450g finnan haddie (Scottish smoked haddock), or homemade cold-smoked fish (see note)2 tablespoons 30g unsalted butter1 medium (8-ounce; 225g yellow onion, finely diced)1 pound...