It's a fish pie, using pastry (not mashed potato like many fish pies) but the main ingredient is pilchards (i.e. sardines that have grown up). The fish are gutted, and boned, but the heads, and sometimes tails, are left on. Jane Grigson supplied a recipe here
which is a fairly tradional one but some people, instead of stuffing the fish, make a sauce with the filling ingredients, mustard (hot, spicy English mustard), creme fraiche, and heaven knows what. The key thing is that the heads of the fish poke out between the layers of pastry as though the fish are gazing at the sky. No one knows for sure where the name comes from but that's as good a definition as any. Some people also leave the tails poking out but that's not traditional.