I haven’t really canned, so this was my first try at learning the process. These are not actually canned, just soaked in vinegar. It should last about 3 weeks in the fridge. The Red Long onions of Tropea are about the size of a large shallot, so keep that in mind if you are substituting.
Slice cucumbers and onion to a medium thickness and fennel thinly. Take a colander and place over a bowl to catch brine. Add veggies and toss with salt. Let sit for about an hour. Do not rinse. Pack canning jars with veggies(my first try was with 2 of the tall and skinny 14oz ball canning jars).
In a medium saucepan, add vinegar, sugar, pickling spices, and brine and cook over medium heat – remove just before it starts to boil. Pour mixture over packed veggies to about a 1/2 head space. Cover and let cool for an hour. Place in fridge.