Clean and filet fish, removing pin bones. Cut into serving-sized pieces—anywhere from 4 to 8 oz.
Toast peppercorns, coriander seed and juniper berries in a cast-iron frying pan, over medium heat, until aromatic, about 5 minutes.
Coarsely grind seasonings with a mortar and pestle or in a spice grinder. Mix with salt and sugar in a medium-sized bowl.
Select 2 pieces of fish of about the same size. Lay them out on a sheet of plastic wrap. Press dry rub into each piece so that it’s entirely covered. Place one piece on top of the other, wrap tightly and refrigerate on a baking sheet or a large plate. Repeat until all the pieces of fish are rubbed and wrapped.
Leave fish in the refrigerator for 1 hour to 90 minutes, for salmon; and 1 hour, maximum, for trout. (By this time, the rub will have dissolved in the liquid from the fish.)
Unwrap fish and rinse each one lightly under running water, leaving some spices on the flesh. Pat dry, with paper towel, and place in a single layer on baking sheets. Return to the fridge to dry for 1 to 2 hours, until the flesh feels tacky to the touch.
Heat the smoker to 170°F and allow it to fill with smoke. Oil the smoker trays and arrange fish on trays according to size—bigger pieces together.
Place trays in the smoker with the thicker pieces on the lower shelves and the thinner ones on the higher shelves. Brush with birch syrup every 30 minutes.
Smoke until the internal temperature reaches 120°F for a wetter texture, or up to 160°F for a drier, more chewy texture.
Remove from the smoker, cool, package, label and freeze. Will keep well for up to 6 months in the freezer.
OR
Add all ingredients to a medium-sized bowl. Whisk until sugar and salt have dissolved.
Arrange trout and salmon pieces in a single layer, flesh-side down, in a large glass, porcelain or stainless-steel pan. Alternatively, lay pieces flat in a large resealable bag on a baking tray.
Pour brine over top of fish in the pan, lifting each piece to make sure it runs underneath. If using a resealable bag, pour brine into a measuring cup, seal the bag, except for one corner, and pour brine into the gap. Seal, lay flat and, if necessary, manipulate the fish pieces so they’re laying flat, flesh side down.
Leave fish in brine for 8 hours or overnight. Remove fish from brine and pat dry without rinsing. Arrange on baking trays and return to the fridge to dry for at least one hour and up to four hours, until flesh feels tacky.
Follow smoking instructions above.