For the turkey: Preheat the oven to 325 degrees F.
Remove the neck and giblets from the turkey (reserve for another use) and place the turkey breast-side up in a roasting pan. Remove 1 tablespoon of the onion soup mix and mix with the butter. With your fingers, gently loosen the skin of the turkey breast and rub the butter under the skin. Sprinkle the remaining soup mix all over the turkey and gently rub. Pour the broth into the bottom of the roasting pan. Place the onion and lemon in the cavity of the turkey.
Roast for 12 to 15 minutes per pound, basting occasionally and tenting the turkey if it starts to get too brown, until a thermometer inserted in the thickest part of the thigh registers 165 degrees F, 3 hours to 3 hours 45 minutes. Remove the turkey to a platter and loosely tent with foil while you make the gravy.
For the gravy: Pour off all of the drippings into a bowl or fat separator. Return the roasting pan to low heat. Add in about 1/3 cup or so of the fatty drippings and discard the rest of the fat, saving the remaining liquid. Sprinkle in the flour and cook, stirring, until the flour is incorporated and bubbly, 1 to 2 minutes. Slowly whisk in the roasting liquid and bring to a simmer. Add enough of the chicken broth to reach the desired consistency and volume. Simmer until slightly thickened, a few minutes. Taste before adding any salt, as the onion soup mix is salty.
Carve the turkey and serve with the gravy on the side.