My first duty is to admit uncertainty in relation to the spelling of this dish. My mother spells it "Jansen's"; but I have seen it claimed that the name originated with the opera singer Pelle Janzon; alternatively that it is Jansson's delight, borrowed from a film called Janssons frestelse or Jansson's temptation. Whichever, I set out my mother's recipe below: I note that the ingredients call for single cream but the method refers to milk.
1 lb potatoes
2 large onions
2 tins anchovies
Cut potatoes & onions to matchstick size. Layer with pieces of anchovy fillets in baking dish. Sprinkle with residue of tin. Salt & pepper & pour in milk. Top with breadcrumbs and bake until potatoes are tender. Sardines, herrings etc may be substituted for anchovies.