Combine the cucumbers, onion, and 2 tablespoons salt in a colander. Let sit for 30 minutes. Transfer to a dish towel and squeeze to remove as much water as possible.
Combine the cucumber mixture, vinegar, sugar, mustard seeds, celery seed, and turmeric in a medium pot. Simmer over medium-high heat, stirring occasionally, until the cucumber is softened slightly, 4 to 6 minutes.
Transfer the mixture to a bowl, cover, and refrigerate until cold, at least 3 hours and up to 5 days.