Removed the sardines from the cans, set aside. Drain the oil from the cans into a large skillet and heat over medium heat. Add the onions, fennel and garlic and cook, stirring occasionally, until the vegetables are very soft but not browned (about 5 minutes). Season with salt and pepper. Place the onion relish on a platter and keep warm. Add the white wine to the pan and simmer 2 or 3 minutes. Add the sardines and cook until pan juices on top of the relish.