I like a lot of different types of fish - doesn't matter which ones too much but I like to include some prawns, some white fish, some smoked fish and whatever else. Sometimes mussels or cockles or salmon. I poach in 50:50 milk:water then save the poaching liquid to make a sauce thickened with sauce flour and then add fresh chopped parsley, sixthed hard-boiled eggs and peas.
I prefer peas served on the side, but the rest of the family like them in with the sauce. Then I top with a THIN layer of mashed potato (mashed without butter) and bake. Have never been one to top with cheese.
I tried the Jamie Oliver one which involved grating loads of veg and thought it was AWFUL, but he did another lovely one with a really lemony sauce and spinach in the pie and that was nice.