In a small skillet over medium heat toast the peppercorns, coriander seeds, cumin seeds and fennel seeds until very fragrant, about 3 minutes. Let cool, then finely grind in spice grinder or with mortar and pestle.
Transfer to a small bowl and mix with kosher salt, sugar, citrus zest and grated beets. Set aside.
Rinse salmon with cold water and pat dry salmon with paper towels. Using needle nose pliers, remove any pin bones you may feel by running your hand over salmon. Pull and discard.
Lay 2 sheets of plastic wrap on counter, slightly overlapping. Spread half the beet salt cure down and then lay salmon, skin side down, on top. Pack remaining beet salt cure on top, pressing to adhere. Bring up the sides of the plastic wrap and wrap around the fish. Wrap once more to secure and seal completely.
Place wrapped salmon in a deep dish or rimmed baking tray. Set another smaller dish on top of salmon and weight it down with cans, bricks or something heavy.
Place in the refrigerator for at least 2 days, or up to 5 days to cure.
Unwrap salmon and rinse under cool running water, running hand and fingers over salmon to remove caked beet salt cure and spices. Pat dry with paper towels.
Transfer salmon to cutting board, skin side down. Using a very sharp knife, thinly slice holding the blade at a diagonal.
Serve with these optional ideas: pumpernickel bread, toasted bagels, cream cheese, capers, chopped or thinly sliced red onions, sliced tomatoes and or dill sprigs or sprouted greens.
Notes
Wrap and refrigerate leftovers, will keep for a week. If you made this recipe with fresh (not frozen) salmon, you can tightly seal slices or filets with plastic wrap and freeze for up to two months.