A simple Bechamel sauce might work
http://www.bbc.co.uk/food/recipes/bechamelsauce_70004, I might use a bay leaf as well as parsley (bay is one of my favourite herbs) and I stud my onion with one or two cloves as well and add a grating of nutmeg if I don't have mace, but mace is best for this and it keeps for ages in a dark glass jar (like a very well washed Bovril/Marmite one) in a dark cupboard. I also tend to leave the flavourings standing in the milk for at least half an hour before I'm ready to make the sauce, it gives a more intense flavour. Those with wheat/gluten problems can thicken the sauce with slaked cornflour and use a mix of cream & milk to replace the richness of the butter.
Pistachio & Parmesan suggest Italy to me so a Mediterranean tomato sauce made by thinly slicing onion and softening it in olive oil, adding some crushed garlic and cooking for a minute or so more, add some thinly sliced peppers (lots of different colours looks good) and fry for a minute or so, add a tin of chopped tomatoes, season and add a pinch of oregano(and/or a bay leaf), bring to boil, cover and simmer for 10 - 20 minutes then uncover and reduce to a sauce consistency; you don't want the onions and peppers to cook to a mush but rather look like multi coloured spaghetti in a tomato sauce. You could add some celery and/or fennel slices with the onion and forget the peppers, or keep all of it in for something more like a Mediterranean vegetable stew.
Alternatively a parmesan flavoured cheese sauce might fit well, don't use all Parmesan in the sauce, it may well be too strong, but mix it with a milder flavoured cheese.
Fish doesn't want a lot of cooking, I find many recipe cards tend to weigh too heavily on the "It must be properly cooked otherwise we'll get sued because of food poisoning" end of cooking times and so the fish ends up like wadges of cotton wool. To get a nice browned crust on a crusted, baked fish dish bang it under a hot grill for a few seconds or wave a blow torch over it.
Currently investigating the Hillmans of Sussex.