Kosha means to fry-and-stir slowly over low heat, working the masala until the oil separates and comes back and the whole thing darkens. Applied to goat meat on the bone, over roughly two hours, it produces something closer to a lacquer than a curry.
Mustard oil, whole garam masala, a large quantity of onion, ginger and garlic paste, a little sugar caramelised at the start for colour. No tomato in the orthodox version, no cream in any version.
It is Sunday food. In a Bengali household this is what the man of the house traditionally cooked, badly and at length, while everyone waited.
Eat it with luchi in the morning or with plain rice at two in the afternoon, then sleep.
