To make the rice: In a 4 to 6-quart saucepan, heat 2 tablespoons olive oil until just smoking. Add half the chopped red onion and saffron, and cook until softened, about 7 to 9 minutes. Add rice and chicken stock, and bring to boil, uncovered. Cook until liquid is absorbed and rice is tender, about 25 minutes. Spread rice out to cool on a large cookie sheet, about 1/4-inch to 1/2-inch thick. When rice is cool, mix with the Pecorino, 2 eggs, and 2 egg yolks, and set aside
To make the stuffing: In a 10 to 12-inch saute pan, cook the remaining chopped onion in 2 tablespoons olive oil until light golden brown, about 10 minutes. Add the tomato sauce, chopped sardines, and mint, and cook 5 to 7 minutes until reduced by one third and quite thick. Set aside and allow to cool.
To assemble: Place the flour for dusting, egg whites, and breadcrumbs in 3 separate shallow bowls and lightly beat egg whites. To form arancine, place 3 tablespoons rice mixture in palm of hand and make an indentation with the back of a spoon. Spoon 2 tablespoons sardine mixture into indentation and form an open bowl out of the rice in your hand. Cover filled portion with another tablespoon of rice mixture and using both hands, shape the rice into a ball. Continue with remaining rice mixture and stuffing until done.
Roll each ball in flour, then egg whites, then breadcrumbs and refrigerate 1 hour, uncovered.
Heat 4 inches of cooking oil in fryer or deep frying pan to 375 F degrees. Carefully place 3 balls at a time into pan and fry until golden brown, about 3 to 4 minutes, stirring with tongs or kitchen spoon to keep them moving. Drained cooked balls on paper towels and place on large platter and serve either warm or at room temperature.