Thinly slice the red onion and toss with kosher salt, lemon juice, and zest. Let sit for 10 minutes.

Drain and rinse the butter beans and chickpeas. Add them to the bowl with the marinated onion and add chopped olives, mint, and parsley. Gently fold together.

Heat the olive oil in a small pan, add minced garlic and sauté until golden. Stir in za’atar, sumac, and cumin.

Drizzle the warm marinade over the salad mixture and gently toss to combine.

Transfer the salad to an airtight container and refrigerate for at least two hours before serving.