Place cucumber slices in a colander set in the sink. Sprinkle with salt; stir to combine. Let stand for 20 minutes. Rinse, drain and transfer to a large heatproof bowl.Advertisement
Meanwhile, combine cider vinegar, white vinegar, brown sugar, onion, garlic, dill and mustard seed in a medium saucepan. Bring to a boil. Reduce heat and simmer for 10 minutes. Pour the hot liquid over the cucumbers; stir to combine. Refrigerate for at least 10 minutes to bring to room temperature.
Make Ahead Tip: Cover and refrigerate for up to 10 days.