Start by thinly slicing the red onion and placing it in a large bowl. Add lemon juice, lemon zest, and salt, then toss well. Let it sit for several minutes to soften and mellow the sharp flavor.
Drain and rinse the chickpeas and butter beans thoroughly. Add them to the bowl along with chopped olives, artichoke hearts, mint, and parsley. Toss gently so the ingredients are evenly distributed.
Heat olive oil in a small skillet over medium heat. Add the chopped garlic and cook briefly until lightly golden and fragrant. Stir in za’atar, sumac, and cumin, then immediately remove from heat to prevent burning.
Pour the warm marinade over the bean mixture. Toss thoroughly so every bean is coated with the flavorful oil. Let the salad marinate for at least two hours in the refrigerator for best flavor, though it can be served immediately if needed.