Place dill leaves in the food processor and pulse until roughly chopped..
Zest grapefruits (or oranges) and lemons. Save fruit for another use.
Add citrus zest to the food processor along with the salt, brown sugar, and peppercorns and process until combined.
Place parchment paper on a large rimmed baking sheet and put one-third of dill cure lengthwise down the center of parchment.
Lay salmon skin-side down (do not rinse salmon) on top of dill cure and pack remaining cure in an even layer onto flesh side.
Fold parchment up and over fish and cover with another sheet of parchment paper. Wrap tightly with plastic or put in a gallon-size zip-loc bag. Place another large rimmed baking sheet on top of fish and set heavy cans or a heavy pot on the baking sheet to weigh down.
Chill 3 days, undisturbed.
Unpack fish and wipe off cure with a damp paper or kitchen towel (do not rinse).
Make sure fish is cold before slicing. Place fish on a large cutting board and slice very thinly with a long, thin, sharp knife. (See Notes below).
Slice at a 45-degree angle to get wide ribbons (the first few slices won't be perfect—keep going!). There are also many Youtube videos out there on “how to slice lox” if you need extra help.
Arrange salmon on a platter and serve with your favorite fixings and spreads.