Add the drained chickpeas to a large bowl. In a small bowl, combine the garlic and Aleppo pepper.
Lightly crush the coriander seeds by either blitzing once or twice in a spice grinder; using a mortar and pestle; or crushing with the back of a sturdy mug.
Toast the spices: Heat the olive oil in your smallest frying pan or saucepan over medium heat for 2 minutes. Add the crushed coriander and cumin seeds. Allow to sizzle and swirl the pan frequently to prevent burning. Cook for 2 minutes, or until the spices are a few shades darker and very aromatic. Immediately pour the hot oil over the garlic mixture and allow to sizzle. Stir, then let sit for 1 minute.
Pour the infused oil over the chickpeas. Stir in the minced preserved lemon (or 1 TBSP lemon zest), 2 TBSP (30 mL) lemon juice, and sumac, if using. Season generously with salt and pepper (I use ~ 1 ½ tsp kosher salt). Set aside to marinate.
Add the sliced red onion to a bowl of ice water (to mellow out the pungency). Soak for 10 minutes, then drain and pat dry.
Dice the cucumbers. Finely chop the parsley and mint.
Tahini Dressing: In a bowl, combine the tahini, 3 TBSP (45 mL) ice water, 2 TBSP (30 mL) lemon juice, maple syrup, and ½ teaspoon kosher salt. Stir with a fork until thick and very creamy (I like mine spreadable and thick, but if using less tahini, it will be a bit thinner). Taste, adding lemon juice or salt to taste, or more maple syrup for sweetness.
Add the cucumbers, drained onions, sesame seeds, and half of the mint/parsley to the chickpea salad. Toss well to combine. Taste, adding more lemon juice or salt as desired.
Schmear the Tahini Dressing onto the bottom of a shallow bowl or plate. Top with the chickpea salad and the remaining mint and parsley. Sprinkle the salad with a bit of sumac and Aleppo pepper and flaky sea salt to taste.