In a large bowl combine the sliced cucumbers with the salt, stir to combine and refrigerate for 1-2 hours. Rinse the cucumbers to remove the excess salt, drain, return to the bowl and toss together with the sliced onion. Divide the cucumbers and onions between two 1-quart jars.

In a medium non-reactive saucepan (stainless steel or similar) combine the vinegar, sugar, apple cider vinegar, pickling spice, mustard seed and turmeric. Bring to a simmer over medium heat and whisk just until the sugar has dissolved. Remove from the heat and cool slightly.

Pour the warm brine over the cucumbers and onion to cover. Let cool to room temperature, cover, and refrigerate for 2-3 days or up to 1 month.