To make the chickpeas: preheat your oven to 425˚F. Pat your chickpeas dry in a clean tea towel and place them on a sheet tray. Drizzle with olive oil and toss until the chickpeas are well coated. Transfer to the oven and bake until the chickpeas are crispy, 25 to 30 minutes.
The chickpeas should appear darker and smaller in color, make a rattling noise when the pan shakes, and easily crush as they cool. If they still have a bit of softness to them, return them to the oven and bake for another few minutes.
While the chickpeas are roasting, place the cumin and coriander seeds in a dry pan over medium heat. Toast, shaking the pan often until the spices are browning and fragrant. Transfer from the pan and crush the seeds using a mortar/pestle or spice grinder. I like to leave the spices in slightly bigger pieces, but you can also fully grind them into a powder. Transfer to a medium bowl and stir in the sumac, lemon zest, and salt.
Once the chickpeas are done, let them cool slightly, then crush using the same mortar/pestle. Alternatively, place them on a cutting board and roll over them with a rolling pin. Add the chickpeas, while still slightly warm, to the spices. Stir until well combined.
To make the salad: Cut the cucumber into ½" pieces and place in a medium bowl. Halve or quarter the cherry tomatoes and add to the cucumbers and red onion. Stir in the crushed chickpeas and finish with the olive oil and lemon juice; taste and add more if desired. Sprinkle with more sumac before serving.