According to tradition Napoleon demanded a quick meal after the battle and his chef was forced to work with the meager results of a forage: a chicken (and some eggs), tomatoes, onions, garlic, herbs, olive oil, and crayfish. The chef cut up the chicken (reportedly with a sabre) and fried it in olive oil, made a sauce from the tomatoes, garlic and onions (plus a bit of cognac from Napoleon's flask), cooked the crayfish, fried the eggs and served them as a garnish, with some of the soldier's bread ration on the side. Napoleon reportedly liked the dish and (having won the battle) considered it lucky. He refused to have the ingredients altered on future occasions even when his chef tried to omit the crayfish.
Modern versions of the dish are made by first flouring then browning chicken portions in oil or butter. The part-cooked chicken is then transferred into a tomato sauce (usually made with onions, garlic, wine & chopped tomatoes). The whole is then cooked slowly until the chicken is done, and a few minutes before serving, a good amount of chopped herbs and black olives are added. This would usually be eaten with a potato dish of some sort, or just crusty bread.
According to tradition, Napoléon's chef was simply using the ingredients he had in hand. In reality, olive trees did not grow in the then-relatively cold climate of Piedmont, where the city of Spinetta Marengo is located, even though olive oil imported from neighboring Liguria is very common.
It seems, that the people from Pakistan, did some modifications (i.e. green chilli) to the dish and named it Karahi Gosht, which is a very famous dish between Pakistani people and Indian people.