Place the onion rings in a medium bowl. Combine the wine, vinegar, bay leaf and a good grinding of pepper in a small saucepan and bring to the boil. Pour the liquid over the onions and leave to marinate for 1 hour. Drain.
Toast the bread. Divide the onions among the bread slices and arrange 2 sardines on top of each. Sprinkle with the parsley and add a thin slice of lemon.