Bitter melon
An elongated green gourd with warty skin, crisp when raw and distinctly bitter.
Bitter melon is the unripe fruit of a climbing plant in the gourd family; in Japan, especially on Okinawa, it is called goya. It has a light to dark green, ridged and warty skin, and inside a white, spongy pith with seeds, which is scraped out before cooking. The flesh is crisp and juicy, and the bitterness is not a background note but the heart of its flavour. To soften it, slice the melon thinly, salt it and let it stand briefly before squeezing it out or blanching it. It is sold fresh in Asian grocers and in Indian and Pakistani shops, and in summer sometimes at vegetable markets.
Substitute:
There is no real substitute, because the bitterness is the reason bitter melon is in the dish. Courgette or green pepper match the bite but strip the dish of its character, so the result is a different dish rather than a variation.