Add the vinegar, 2 cups water, mustard seeds and pickling spices into a large pot and bring to a boil. Drain the cucumbers, add to the pickling brine and when the pot has just come to a second boil, turn off the heat. Slice 1 large onion in half, then in very thin slices. Add about a teaspoon of salt (for a two cup car, less for smaller jars) into the bottom of jar, then scoop in the cukes, lay thinly sliced onions on top, and pour syrup over. Tightly close the jars and let cool to room temperature, then store in the fridge for at least a week.
These will keep in the refrigerator for quite a while, but once you try them, I doubt they’ll last long!