Season both the outside and inside of the duckling with salt and pepper. Place the onion inside the duck. Truss the bird and prick the skin. Dry thoroughly.
Place the duck breast-side up in a roasting pan and place in the center rack of a 425 degree preheated oven for 15 minutes to brown lightly. Reduce the oven temperature to 350 degrees and roast for 1 additional hour to 1 hour and 25 minutes. Remove accumulated fat occasionally with a bulb baster.
The duck is done to medium rare if the juices from the fattest part of the thigh or drumstick run faintly rosy when the duck is pricked, and when the duck is lifted and drained, the last drops of juice from the vent are pale rose. The duck is well done when the juices run pale yellow.
Remove the duck from the oven, discard trussing strings, and place on a serving platter. Let sit for 10 minutes before carving.
In a saucepan, combine the red currant jelly and the lemon juice. Bring to a boil, strain, and serve with the duck.