Heat vegetable oil in a large frypan over medium heat. Saute onions until soft, but not browned. Remove from pan using a slotted spoon and set aside. Slice the liver into serving size pieces and remove any membrane.
In a shallow dish, mix together the flour, salt and pepper. Coat the liver with the flour mixture and place into the hot frypan. Brown the liver quickly on each side. Place reserved onions on top of liver pieces. Pour the passata and water over everything in the pan, lifting the liver pieces to let the tomato sauce coat the bottom of the pan.
Sprinkle the oregano over everything and simmer over low heat for 8 to 10 minutes, stirring occasionally. Adjust salt and pepper to taste. Be careful not to overcook or liver will be tough. I serve this with mashed potatoes and green beans.