In a large colander, toss the cucumber and onion with 2 tablespoons of the salt and let stand for 10 minutes. Rinse the cucumber and onion and drain well.Meanwhile, in a medium saucepan, combine the remaining 1 tablespoon of kosher salt with 1 cup of water and the vinegar, sugar, jalapeño, celery seeds, turmeric and horseradish; bring just to a boil, stirring to dissolve the salt and sugar. Remove the brine from the heat. Add the cucumber and onion and let cool completely; you should have a total of 2 cups.
Transfer the cucumber and onion with the brine to the canister of a 1-pint whipped cream dispenser. Seal the siphon and charge it with one cream (N20) cartridge according to the manufacturer’s instructions. Shake the siphon to distribute the gas and refrigerate for 20 minutes.
Hold a measuring cup upside down over the nozzle to catch any liquid. With the siphon in an upright position, very gently press the handle to release the gas. When all of the gas has been released, unscrew the top. Pour the pickles into a bowl or jar and serve.
The pickles can be refrigerated in their brine for up to 2 weeks.
Siphon Tip To get the pickles out of the siphon, release the gas: Hold the siphon upright, with a cup shielding the nozzle, then gently press the handle. Open the canister and pour out the pickles with their brine.