In a dry pan over medium heat, toast the mustard seeds until fragrant, about 2 minutes. Set aside.
Add the cucumbers to a medium bowl. Season with three large pinches of salt and set aside for 10 minutes. In the meantime, combine the red onion, yogurt, lemon juice, garlic, dill and mustard seeds in a large bowl and stir to combine.
Pour off any excess liquid that has accumulated in the cucumber bowl and pat the cucumbers dry. Add the cucumbers to the yogurt mixture, mix well, and season to taste with salt and pepper.