Using a mandolin or sharp knife, thinly slice onions and cucumbers into very thin rounds. Place into a large bowl, sprinkle over the salt. Toss gently to coat. Cover and set aside 30 minutes.
Tip the mustard seeds into a small, dry frying pan. Place over medium heat and cook, shaking the pan 3-4 minutes until warm and aromatic.
Combine the water, vinegar, sugar, pepper and warm mustard seeds in a large jug, stir to dissolve the sugar.
Using your hands, squeeze the onions and cucumber, in batches, over the sink to remove excess liquid. Place the onions and cucumber into a tall, clean jar or deep glass bowl. Pour over the pickling mixture, making sure the vegetables are covered. Refrigerate overnight.
Spoon onto a shallow bowl, scatter with mint leaves and serve.