1) In a saucepan, melt butter.
2) Add in the onions and saute till they become tender.
3) Mix in the condensed cream of mushroom soup, water, cheese and sherry.
4) Cook the pan on low heat till the cheese has melted. Keep stirring.
5) Add in spaghetti, cooked chicken or turkey and pimiento. Mix well and cook till all the ingredients are heated through.
6) Sprinkle the Party Tetrazzini with chopped parsley leaves.