1. To make the sumac onions, in a small bowl mix the onions with 1 tablespoon olive oil, 1 1/2 teaspoons sumac, and salt. If the onions seem a little dry, add a touch more olive oil until they are glistening. Set aside.
2. Put the diced cucumbers in a strainer or colander, sprinkle with 1/4 teaspoon salt and let stand over a bowl for about 10 to 15 minutes, to drain away some of the liquid. (Don’t let them sit too long, though, as you don’t want them to get soggy.)
3. Put the tomatoes, radishes, mint, parsley, thyme, and crumbled flatbread in a large bowl. Blot the drained cucumbers with a paper or kitchen towel, then add them along with the buttermilk, 1/4 cup olive oil, lemon juice, garlic, vinegar, 1 1/4 teaspoons salt, 1 1/2 teaspoons sumac, half the sumac onions, and several good grinds of black pepper.
4. Mix everything together then transfer to a serving platter. Strew the remaining sumac onions over the salad, drizzle with a little extra olive oil, and sprinkle with the remaining 1/2 teaspoon of sumac. Serve immediately.