Heat the oil in a large, heavy skillet. Add the onions and bay leaves and cook over medium-low heat until the onions are golden. Season with salt and pepper and add the water.
Remove from the heat.
Transfer the onions to another dish, draining as much oil as possible back into the pan and set aside.
Lightly dust the pieces of liver with the flour.
Heat the oil remaining in the skillet to medium-high. Add the pieces of liver and stir-fry until they are seared.
Return the onions to the skillet, remove the bay leaves, and reheat the onions and liver together.
Stir in the vinegar, season with salt and pepper, and serve with white polenta on the side.