Peel and cut the onion into eighths, and put into a freezer bag with the oil, mustard, dried sage, a good grinding of pepper, and Worcestershire sauce. Cut lemon in half, squeeze juice into bag, and then cut the halves into eighths and add them. Squidge everything around to mix (the mustard needs help to combine) and then add the chicken pieces. Leave to marinade in the refrigerator overnight, or for up to 2 days.
Preheat the oven to 425 degrees F. Allow the chicken to come to room temperature in its marinade.
Arrange the chicken pieces in a roasting tin skin side up with the marinade, including all the bits and pieces, and tuck the sausages around them. Sprinkle the fresh sage leaves over the chicken and sausages and then put the tin into the oven to cook for 1 hour and 15 minutes. Turn the sausages over half way through to color them evenly.
Arrange the chicken and sausages on a large platter.