Smoked haddock gratin, with spinach & peas

My favourite smoked haddock, from @alfredenderby (I’ve got a stash in the freezer too)⁠. If you don’t believe me, just ask Marco Pierre White or the Great Taste Award judges 🐟🐟🐟⁠ 750 g smoked haddock, cut into 4 pieces⁠ 500 ml milk⁠ 2 bay leaves⁠ ½ onion, peeled⁠ 30 g butter⁠ 30 g plain flour⁠…