Start by preheating your oven to 400°F.
On a baking sheet lined with parchment paper, spread out the drained chickpeas. Drizzle them with olive oil. Roast the chickpeas in the oven for 40 minutes, shaking the pan halfway through to ensure even cooking.
While the chickpeas are roasting, thinly slice half of a white onion. Toss the onion slices in a small bowl with a little olive oil, salt, and pepper. Spread the onions on another parchment-lined baking sheet and roast alongside the chickpeas for 40 minutes, or until they're crispy and golden.
While your crumble topping is baking, prepare the green beans. Wash the beans thoroughly and trim off the ends. Cut the beans in half for easier eating.
In a large skillet, heat a drizzle of olive oil over medium heat. Add the diced onion and sauté until it becomes translucent, about 5 minutes. Add the minced garlic and continue to cook for another 5-10 minutes until the garlic is fragrant and the onions are golden brown.
Once the onions and garlic are golden and aromatic, add the green beans to the skillet. Sauté them together for about 2 minutes to allow the flavors to meld. Stir in the Greek yogurt, freshly grated Parmesan cheese, salt, and pepper. Cook for another 2-3 minutes until everything is well combined and the sauce is creamy and smooth.
Transfer the green bean mixture to a greased casserole dish, spreading it out evenly. Once the chickpeas and onions are done roasting, let the chickpeas cool slightly before transferring them to a food processor. Pulse the chickpeas until they reach a flour-like consistency.
In a medium bowl, combine the chickpea flour, crispy onions, grated Parmesan, olive oil, salt, and pepper. Mix until the crumble topping is well combined. Sprinkle the crispy onion chickpea crumble evenly over the top of the green bean mixture. Place the casserole in the preheated oven and bake for 15-20 minutes, or until the top is golden brown and crispy. Enjoy!