Peel and slice the onion thinly. Place in a bowl with ½ teaspoon salt, sumac, and vinegar. Mix well and let sit for at least 30 minutes (or overnight in the fridge).
Slice cucumbers into large, uneven pieces. Place on a cutting board, cover with a clean towel, and gently break them using a rolling pin. Salt with a teaspoon of salt and let rest for 20 minutes.
Drain the cucumbers of excess liquid.
Slice the radishes thinly, preferably with a mandolin or sharp knife.
Separate the leaves of the fresh herbs and add them to the salad along with the rocket.
Mix cucumbers, radishes, pickled onions with their juices, and herbs together.
Drizzle with olive oil and lemon juice. Toss well and adjust seasoning to taste.
Serve and enjoy!