Roast the chickpeas: Drain, rinse and pat dry the chickpeas; let sit 30 minutes on a paper towel–lined plate to let them dry out. Preheat the oven to 350˚ F. Dump the chickpeas onto a rimmed baking sheet. Toss with the olive oil and season with salt and pepper. Roast, tossing occasionally, until crisp and dry, about 50 minutes. Let cool.
While the chickpeas cool, bake the croutons: Toss the pita pieces with the butter and a pinch of salt on another baking sheet. Bake until golden brown, about 10 minutes. Let cool. (You can store the chickpeas and the croutons in separate containers at room temperature for up to 4 days.)
Make the dressing: Mix the tahini, lemon juice and 3 to 4 tablespoons water in a bowl until creamy and pourable. Season with salt and pepper. (You can put the dressing in a cute jar and store it in the fridge for up to 5 days. If it gets too thick to pour, mix in a little water before using.)
Make the salad: Combine the spinach, tomatoes, cucumber, onion, radishes and mint in a large bowl. Add the feta, za'atar, dressing and chickpeas and gently toss. Top with the croutons.