Thaw the trout if frozen, then rinse thoroughly under water. Ensure they are gutted with gills removed. Leave scales on.
Dissolve salt in hot water from a kettle. Add roughly mustard seeds, some sage, a few bay leaves, and pepper to taste. Cool the mixture with cold water to the proper ratio.
Place the trout in the cooled brine and refrigerate for 12 hours. (Leaving them for up to 13 hours has minimal negative effects.)
Remove the trout from the brine and rinse thoroughly. Pat dry with paper towels until the skin feels slightly wrinkled like paper.
Line the bottom of the smoking oven with aluminum foil strips. Wrap the drip tray with aluminum foil. Add 2-3 tablespoons of smoking flour and roughly 10 juniper berries to the bottom tray. Fill the alcohol burners according to the oven's instructions.
Arrange the trout on the oven grids, ensuring they do not touch each other.
Light the alcohol burner. Smoke the trout at 60-90 °C (140-194 °F) for 15-20 minutes. If the temperature exceeds 90 °C, briefly remove from heat or reduce the burner.
Check doneness by pulling the dorsal fin from one trout; it should come out easily. If not, smoke for another 5 minutes and check again.