Mouri Bata Koi Mach is a traditional Bengali fish curry made with koi fish and a flavorful mustard and fennel seed paste. The fish is marinated with turmeric powder and salt, then fried until golden brown. The mustard and fennel seed paste is made by grinding together mustard seeds, fennel seeds, green chilies, and other spices. The fried fish is then cooked in this paste, resulting in a delicious and aromatic curry. It is best served with steamed rice.