Mix the salt, sugar, dill, lemon zest and grapefruit zest together in a bowl. Line a platter or small baking pan with plastic wrap and spread about a third of the salt mixture on top of the plastic in the general size and shape of the salmon. Lay the salmon on top and make 5 to 6 shallow slits into the flesh using the tip of a paring knife, then rub the orange liqueur on top. Sprinkle evenly with the remaining salt mixture and wrap the plastic over the fish to cover it completely. Place the wrapped salmon inside a ziptop bag, remove as much of the air as possible and seal the bag. Put a plate on top of the wrapped fish and weigh it down with a small can or something similar. Refrigerate for 24 hours (the fish will release some liquid as it sits).
After 24 hours, scrape the salt mixture off the top of the fish and wipe the surface with a damp paper towel. Serve immediately or wrap in plastic and keep refrigerated up to 5 days.
When ready to serve, use a very sharp knife to make thin slices on a bias. Serve with the Lemon Creme Fraiche, toast points, red onions and lemon wedges.
Lemon Creme Fraiche:
Combine the creme fraiche, lemon zest and juice and chives in a small bowl; season with salt.