Gently smash cucumbers with a rolling pin or the back of a heavy knife just to break open. Tear into irregular 2-inch pieces and place in a medium bowl; season lightly with salt. Let sit at least 5 minutes and up to 1 hour.
Meanwhile, smash the garlic. Place in a medium jar and marinate garlic in the olive oil along with red pepper flakes and lemon juice. For best results, let marinate overnight.
Drain cucumbers, discarding any liquid they have released. Add to bowl along with chopped tomatoes and spring onions. Pour the dressing over the cucumbers and tomatoes and toss several times to coat. Top with feta, za’atar or parsley, and sesame seeds.