Very finely chop the preserved lemon peel so that it resembles a paste. Add it to a large zip-top plastic bag along with 2 tsp of the preserved lemon liquid from the jar and the olive oil.
To the bag, add the garlic, parsley, oregano, date syrup (or honey), harissa, cumin, paprika, coriander and generous grind of black pepper. Seal the bag and squish everything until well combined.
Add the salmon to the bag with the marinade and seal tightly. Squish the marinade to ensure that it's completely covering the salmon. Allow to rest at room temperature for 15-20 minutes or in the refrigerator for no more than 1 hour.
Preheat oven to 180°C/350°F. Remove the salmon from the marinade, shaking off the excess. Add to a parchment-lined baking sheet and move to the oven. Cook until the internal temperature registers at 50°C/125°F when an instant-read thermometer is inserted into the thickest part of the fillet - about 15 minutes. Allow to rest for 5 minutes before serving.